diff --git a/go.mod b/go.mod
index ed6dc4b..0226986 100644
--- a/go.mod
+++ b/go.mod
@@ -4,7 +4,7 @@ go 1.20
 
 require (
 	github.com/google/go-cmp v0.6.0
-	github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231211173412-2017fc8cadb4
+	github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231212192300-fb1d856be38c
 	github.com/hashicorp/terraform-plugin-log v0.9.0
 	google.golang.org/grpc v1.59.0
 )
diff --git a/go.sum b/go.sum
index 03a586e..8977863 100644
--- a/go.sum
+++ b/go.sum
@@ -16,8 +16,8 @@ github.com/hashicorp/go-plugin v1.6.0 h1:wgd4KxHJTVGGqWBq4QPB1i5BZNEx9BR8+OFmHDm
 github.com/hashicorp/go-plugin v1.6.0/go.mod h1:lBS5MtSSBZk0SHc66KACcjjlU6WzEVP/8pwz68aMkCI=
 github.com/hashicorp/go-uuid v1.0.3 h1:2gKiV6YVmrJ1i2CKKa9obLvRieoRGviZFL26PcT/Co8=
 github.com/hashicorp/go-uuid v1.0.3/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
-github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231211173412-2017fc8cadb4 h1:Q3xOtc6IB248qD76gKYD6FEPD0eFYKKr9n18Zh5o5I0=
-github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231211173412-2017fc8cadb4/go.mod h1:dYHQT4x4qtYsGsD0ZmWo0JJz9rIxSv/ETdK8p+1qlhI=
+github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231212192300-fb1d856be38c h1:+/OS6VVVQZ3mdF1H2KbMYxKp329S61MDbNLQs4L1eCg=
+github.com/hashicorp/terraform-plugin-go v0.19.2-0.20231212192300-fb1d856be38c/go.mod h1:dYHQT4x4qtYsGsD0ZmWo0JJz9rIxSv/ETdK8p+1qlhI=
 github.com/hashicorp/terraform-plugin-log v0.9.0 h1:i7hOA+vdAItN1/7UrfBqBwvYPQ9TFvymaRGZED3FCV0=
 github.com/hashicorp/terraform-plugin-log v0.9.0/go.mod h1:rKL8egZQ/eXSyDqzLUuwUYLVdlYeamldAHSxjUFADow=
 github.com/hashicorp/terraform-registry-address v0.2.3 h1:2TAiKJ1A3MAkZlH1YI/aTVcLZRu7JseiXNRHbOAyoTI=
diff --git a/tf5muxserver/mux_server_CallFunction.go b/tf5muxserver/mux_server_CallFunction.go
index c72bdb2..7e4ec8c 100644
--- a/tf5muxserver/mux_server_CallFunction.go
+++ b/tf5muxserver/mux_server_CallFunction.go
@@ -30,7 +30,28 @@ func (s *muxServer) CallFunction(ctx context.Context, req *tfprotov5.CallFunctio
 	}
 
 	ctx = logging.Tfprotov5ProviderServerContext(ctx, server)
+
+	// Remove and call server.CallFunction below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := server.(tfprotov5.FunctionServer)
+
+	if !ok {
+		resp := &tfprotov5.CallFunctionResponse{
+			Diagnostics: []*tfprotov5.Diagnostic{
+				{
+					Severity: tfprotov5.DiagnosticSeverityError,
+					Summary:  "Provider Functions Not Implemented",
+					Detail: "A provider-defined function call was received by the provider, however the provider does not implement functions. " +
+						"Either upgrade the provider to a version that implements provider-defined functions or this is a bug in Terraform that should be reported to the Terraform maintainers.",
+				},
+			},
+		}
+
+		return resp, nil
+	}
+
 	logging.MuxTrace(ctx, "calling downstream server")
 
-	return server.CallFunction(ctx, req)
+	// return server.CallFunction(ctx, req)
+	return functionServer.CallFunction(ctx, req)
 }
diff --git a/tf5muxserver/mux_server_CallFunction_test.go b/tf5muxserver/mux_server_CallFunction_test.go
index d0aea7e..66c1256 100644
--- a/tf5muxserver/mux_server_CallFunction_test.go
+++ b/tf5muxserver/mux_server_CallFunction_test.go
@@ -39,7 +39,15 @@ func TestMuxServerCallFunction(t *testing.T) {
 		t.Fatalf("unexpected error setting up factory: %s", err)
 	}
 
-	_, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+	if !ok {
+		t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+	}
+
+	// _, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 		Name: "test_function1",
 	})
 
@@ -55,7 +63,8 @@ func TestMuxServerCallFunction(t *testing.T) {
 		t.Errorf("unexpected test_function1 CallFunction called on server2")
 	}
 
-	_, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	// _, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 		Name: "test_function2",
 	})
 
diff --git a/tf5muxserver/mux_server_GetFunctions.go b/tf5muxserver/mux_server_GetFunctions.go
index 9090adf..c8927fc 100644
--- a/tf5muxserver/mux_server_GetFunctions.go
+++ b/tf5muxserver/mux_server_GetFunctions.go
@@ -29,9 +29,19 @@ func (s *muxServer) GetFunctions(ctx context.Context, req *tfprotov5.GetFunction
 
 	for _, server := range s.servers {
 		ctx := logging.Tfprotov5ProviderServerContext(ctx, server)
+
+		// Remove and call server.GetFunctions below directly.
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := server.(tfprotov5.FunctionServer)
+
+		if !ok {
+			continue
+		}
+
 		logging.MuxTrace(ctx, "calling downstream server")
 
-		serverResp, err := server.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
+		// serverResp, err := server.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
+		serverResp, err := functionServer.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
 
 		if err != nil {
 			return resp, fmt.Errorf("error calling GetFunctions for %T: %w", server, err)
diff --git a/tf5muxserver/mux_server_GetFunctions_test.go b/tf5muxserver/mux_server_GetFunctions_test.go
index 0c65873..4482913 100644
--- a/tf5muxserver/mux_server_GetFunctions_test.go
+++ b/tf5muxserver/mux_server_GetFunctions_test.go
@@ -311,7 +311,15 @@ func TestMuxServerGetFunctions(t *testing.T) {
 				t.Fatalf("unexpected error: %s", err)
 			}
 
-			resp, err := muxServer.ProviderServer().GetFunctions(context.Background(), &tfprotov5.GetFunctionsRequest{})
+			// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+			functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+			if !ok {
+				t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+			}
+
+			// resp, err := muxServer.ProviderServer().GetFunctions(context.Background(), &tfprotov5.GetFunctionsRequest{})
+			resp, err := functionServer.GetFunctions(context.Background(), &tfprotov5.GetFunctionsRequest{})
 
 			if err != nil {
 				t.Fatalf("unexpected error: %s", err)
diff --git a/tf5muxserver/mux_server_test.go b/tf5muxserver/mux_server_test.go
index e64951d..c5e434c 100644
--- a/tf5muxserver/mux_server_test.go
+++ b/tf5muxserver/mux_server_test.go
@@ -435,7 +435,15 @@ func TestMuxServerGetFunctionServer_GetProviderSchema(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// _, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -498,7 +506,15 @@ func TestMuxServerGetFunctionServer_GetProviderSchema_Duplicate(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function",
 		})
 
@@ -562,7 +578,15 @@ func TestMuxServerGetFunctionServer_GetMetadata(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// _, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -629,7 +653,15 @@ func TestMuxServerGetFunctionServer_GetMetadata_Duplicate(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function",
 		})
 
@@ -691,7 +723,15 @@ func TestMuxServerGetFunctionServer_GetMetadata_Partial(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// _, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -757,7 +797,15 @@ func TestMuxServerGetFunctionServer_Missing(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov5.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov5.FunctionServer")
+		}
+
+		// resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 			Name: "test_function_nonexistent",
 		})
 
diff --git a/tf5to6server/tf5to6server.go b/tf5to6server/tf5to6server.go
index 7158a85..8fae458 100644
--- a/tf5to6server/tf5to6server.go
+++ b/tf5to6server/tf5to6server.go
@@ -44,8 +44,29 @@ func (s v5tov6Server) ApplyResourceChange(ctx context.Context, req *tfprotov6.Ap
 }
 
 func (s v5tov6Server) CallFunction(ctx context.Context, req *tfprotov6.CallFunctionRequest) (*tfprotov6.CallFunctionResponse, error) {
+	// Remove and call s.v5Server.CallFunction below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := s.v5Server.(tfprotov5.FunctionServer)
+
+	if !ok {
+		v6Resp := &tfprotov6.CallFunctionResponse{
+			Diagnostics: []*tfprotov6.Diagnostic{
+				{
+					Severity: tfprotov6.DiagnosticSeverityError,
+					Summary:  "Provider Functions Not Implemented",
+					Detail: "A provider-defined function call was received by the provider, however the provider does not implement functions. " +
+						"Either upgrade the provider to a version that implements provider-defined functions or this is a bug in Terraform that should be reported to the Terraform maintainers.",
+				},
+			},
+		}
+
+		return v6Resp, nil
+	}
+
 	v5Req := tfprotov6tov5.CallFunctionRequest(req)
-	v5Resp, err := s.v5Server.CallFunction(ctx, v5Req)
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	// v5Resp, err := s.v5Server.CallFunction(ctx, v5Req)
+	v5Resp, err := functionServer.CallFunction(ctx, v5Req)
 
 	if err != nil {
 		return nil, err
@@ -66,8 +87,22 @@ func (s v5tov6Server) ConfigureProvider(ctx context.Context, req *tfprotov6.Conf
 }
 
 func (s v5tov6Server) GetFunctions(ctx context.Context, req *tfprotov6.GetFunctionsRequest) (*tfprotov6.GetFunctionsResponse, error) {
+	// Remove and call s.v5Server.GetFunctions below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := s.v5Server.(tfprotov5.FunctionServer)
+
+	if !ok {
+		v6Resp := &tfprotov6.GetFunctionsResponse{
+			Functions: map[string]*tfprotov6.Function{},
+		}
+
+		return v6Resp, nil
+	}
+
 	v5Req := tfprotov6tov5.GetFunctionsRequest(req)
-	v5Resp, err := s.v5Server.GetFunctions(ctx, v5Req)
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	// v5Resp, err := s.v5Server.GetFunctions(ctx, v5Req)
+	v5Resp, err := functionServer.GetFunctions(ctx, v5Req)
 
 	if err != nil {
 		return nil, err
diff --git a/tf5to6server/tf5to6server_test.go b/tf5to6server/tf5to6server_test.go
index 229ad26..1856409 100644
--- a/tf5to6server/tf5to6server_test.go
+++ b/tf5to6server/tf5to6server_test.go
@@ -178,7 +178,15 @@ func TestV6ToV5ServerCallFunction(t *testing.T) {
 		t.Fatalf("unexpected error upgrading server: %s", err)
 	}
 
-	_, err = v6server.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := v6server.(tfprotov6.FunctionServer)
+
+	if !ok {
+		t.Fatal("v6server should implement tfprotov6.FunctionServer")
+	}
+
+	//_, err = v6server.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 		Name: "test_function",
 	})
 
@@ -238,7 +246,15 @@ func TestV6ToV5ServerGetFunctions(t *testing.T) {
 		t.Fatalf("unexpected error upgrading server: %s", err)
 	}
 
-	_, err = v6server.GetFunctions(ctx, &tfprotov6.GetFunctionsRequest{})
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := v6server.(tfprotov6.FunctionServer)
+
+	if !ok {
+		t.Fatal("v6server should implement tfprotov6.FunctionServer")
+	}
+
+	//_, err = v6server.GetFunction(ctx, &tfprotov6.GetFunctionRequest{})
+	_, err = functionServer.GetFunctions(ctx, &tfprotov6.GetFunctionsRequest{})
 
 	if err != nil {
 		t.Fatalf("unexpected error: %s", err)
diff --git a/tf6muxserver/mux_server_CallFunction.go b/tf6muxserver/mux_server_CallFunction.go
index 6804ca3..b600d49 100644
--- a/tf6muxserver/mux_server_CallFunction.go
+++ b/tf6muxserver/mux_server_CallFunction.go
@@ -30,7 +30,28 @@ func (s *muxServer) CallFunction(ctx context.Context, req *tfprotov6.CallFunctio
 	}
 
 	ctx = logging.Tfprotov6ProviderServerContext(ctx, server)
+
+	// Remove and call server.CallFunction below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := server.(tfprotov6.FunctionServer)
+
+	if !ok {
+		resp := &tfprotov6.CallFunctionResponse{
+			Diagnostics: []*tfprotov6.Diagnostic{
+				{
+					Severity: tfprotov6.DiagnosticSeverityError,
+					Summary:  "Provider Functions Not Implemented",
+					Detail: "A provider-defined function call was received by the provider, however the provider does not implement functions. " +
+						"Either upgrade the provider to a version that implements provider-defined functions or this is a bug in Terraform that should be reported to the Terraform maintainers.",
+				},
+			},
+		}
+
+		return resp, nil
+	}
+
 	logging.MuxTrace(ctx, "calling downstream server")
 
-	return server.CallFunction(ctx, req)
+	// return server.CallFunction(ctx, req)
+	return functionServer.CallFunction(ctx, req)
 }
diff --git a/tf6muxserver/mux_server_CallFunction_test.go b/tf6muxserver/mux_server_CallFunction_test.go
index 2b81171..17f6b3f 100644
--- a/tf6muxserver/mux_server_CallFunction_test.go
+++ b/tf6muxserver/mux_server_CallFunction_test.go
@@ -39,7 +39,15 @@ func TestMuxServerCallFunction(t *testing.T) {
 		t.Fatalf("unexpected error setting up factory: %s", err)
 	}
 
-	_, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+	if !ok {
+		t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+	}
+
+	// _, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 		Name: "test_function1",
 	})
 
@@ -55,7 +63,8 @@ func TestMuxServerCallFunction(t *testing.T) {
 		t.Errorf("unexpected test_function1 CallFunction called on server2")
 	}
 
-	_, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	// _, err = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 		Name: "test_function2",
 	})
 
diff --git a/tf6muxserver/mux_server_GetFunctions.go b/tf6muxserver/mux_server_GetFunctions.go
index 91ebfd4..cbb484e 100644
--- a/tf6muxserver/mux_server_GetFunctions.go
+++ b/tf6muxserver/mux_server_GetFunctions.go
@@ -29,9 +29,19 @@ func (s *muxServer) GetFunctions(ctx context.Context, req *tfprotov6.GetFunction
 
 	for _, server := range s.servers {
 		ctx := logging.Tfprotov6ProviderServerContext(ctx, server)
+
+		// Remove and call server.GetFunctions below directly.
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := server.(tfprotov6.FunctionServer)
+
+		if !ok {
+			continue
+		}
+
 		logging.MuxTrace(ctx, "calling downstream server")
 
-		serverResp, err := server.GetFunctions(ctx, &tfprotov6.GetFunctionsRequest{})
+		// serverResp, err := server.GetFunctions(ctx, &tfprotov6.GetFunctionsRequest{})
+		serverResp, err := functionServer.GetFunctions(ctx, &tfprotov6.GetFunctionsRequest{})
 
 		if err != nil {
 			return resp, fmt.Errorf("error calling GetFunctions for %T: %w", server, err)
diff --git a/tf6muxserver/mux_server_GetFunctions_test.go b/tf6muxserver/mux_server_GetFunctions_test.go
index 0168acc..185047f 100644
--- a/tf6muxserver/mux_server_GetFunctions_test.go
+++ b/tf6muxserver/mux_server_GetFunctions_test.go
@@ -311,7 +311,15 @@ func TestMuxServerGetFunctions(t *testing.T) {
 				t.Fatalf("unexpected error: %s", err)
 			}
 
-			resp, err := muxServer.ProviderServer().GetFunctions(context.Background(), &tfprotov6.GetFunctionsRequest{})
+			// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+			functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+			if !ok {
+				t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+			}
+
+			// resp, err := muxServer.ProviderServer().GetFunctions(context.Background(), &tfprotov6.GetFunctionsRequest{})
+			resp, err := functionServer.GetFunctions(context.Background(), &tfprotov6.GetFunctionsRequest{})
 
 			if err != nil {
 				t.Fatalf("unexpected error: %s", err)
diff --git a/tf6muxserver/mux_server_test.go b/tf6muxserver/mux_server_test.go
index 84932a4..d7d68f9 100644
--- a/tf6muxserver/mux_server_test.go
+++ b/tf6muxserver/mux_server_test.go
@@ -435,7 +435,15 @@ func TestMuxServerGetFunctionServer_GetProviderSchema(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		//_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -498,7 +506,15 @@ func TestMuxServerGetFunctionServer_GetProviderSchema_Duplicate(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		// resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function",
 		})
 
@@ -562,7 +578,15 @@ func TestMuxServerGetFunctionServer_GetMetadata(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		// _, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -629,7 +653,15 @@ func TestMuxServerGetFunctionServer_GetMetadata_Duplicate(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		// resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function",
 		})
 
@@ -691,7 +723,15 @@ func TestMuxServerGetFunctionServer_GetMetadata_Partial(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		_, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		// _, _ = muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		_, _ = functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function1",
 		})
 	}
@@ -757,7 +797,15 @@ func TestMuxServerGetFunctionServer_Missing(t *testing.T) {
 	terraformOp := func() {
 		defer wg.Done()
 
-		resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+		functionServer, ok := muxServer.ProviderServer().(tfprotov6.FunctionServer)
+
+		if !ok {
+			t.Fatal("muxServer should implement tfprotov6.FunctionServer")
+		}
+
+		//resp, _ := muxServer.ProviderServer().CallFunction(ctx, &tfprotov6.CallFunctionRequest{
+		resp, _ := functionServer.CallFunction(ctx, &tfprotov6.CallFunctionRequest{
 			Name: "test_function_nonexistent",
 		})
 
diff --git a/tf6to5server/tf6to5server.go b/tf6to5server/tf6to5server.go
index 14fc93f..d7cd394 100644
--- a/tf6to5server/tf6to5server.go
+++ b/tf6to5server/tf6to5server.go
@@ -56,8 +56,29 @@ func (s v6tov5Server) ApplyResourceChange(ctx context.Context, req *tfprotov5.Ap
 }
 
 func (s v6tov5Server) CallFunction(ctx context.Context, req *tfprotov5.CallFunctionRequest) (*tfprotov5.CallFunctionResponse, error) {
+	// Remove and call s.v6Server.CallFunction below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := s.v6Server.(tfprotov6.FunctionServer)
+
+	if !ok {
+		v5Resp := &tfprotov5.CallFunctionResponse{
+			Diagnostics: []*tfprotov5.Diagnostic{
+				{
+					Severity: tfprotov5.DiagnosticSeverityError,
+					Summary:  "Provider Functions Not Implemented",
+					Detail: "A provider-defined function call was received by the provider, however the provider does not implement functions. " +
+						"Either upgrade the provider to a version that implements provider-defined functions or this is a bug in Terraform that should be reported to the Terraform maintainers.",
+				},
+			},
+		}
+
+		return v5Resp, nil
+	}
+
 	v6Req := tfprotov5tov6.CallFunctionRequest(req)
-	v6Resp, err := s.v6Server.CallFunction(ctx, v6Req)
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	// v6Resp, err := s.v6Server.CallFunction(ctx, v6Req)
+	v6Resp, err := functionServer.CallFunction(ctx, v6Req)
 
 	if err != nil {
 		return nil, err
@@ -78,8 +99,22 @@ func (s v6tov5Server) ConfigureProvider(ctx context.Context, req *tfprotov5.Conf
 }
 
 func (s v6tov5Server) GetFunctions(ctx context.Context, req *tfprotov5.GetFunctionsRequest) (*tfprotov5.GetFunctionsResponse, error) {
+	// Remove and call s.v6Server.GetFunctions below directly.
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := s.v6Server.(tfprotov6.FunctionServer)
+
+	if !ok {
+		v5Resp := &tfprotov5.GetFunctionsResponse{
+			Functions: map[string]*tfprotov5.Function{},
+		}
+
+		return v5Resp, nil
+	}
+
 	v6Req := tfprotov5tov6.GetFunctionsRequest(req)
-	v6Resp, err := s.v6Server.GetFunctions(ctx, v6Req)
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	// v6Resp, err := s.v6Server.GetFunctions(ctx, v6Req)
+	v6Resp, err := functionServer.GetFunctions(ctx, v6Req)
 
 	if err != nil {
 		return nil, err
diff --git a/tf6to5server/tf6to5server_test.go b/tf6to5server/tf6to5server_test.go
index 634498d..de8ba35 100644
--- a/tf6to5server/tf6to5server_test.go
+++ b/tf6to5server/tf6to5server_test.go
@@ -276,7 +276,15 @@ func TestV6ToV5ServerCallFunction(t *testing.T) {
 		t.Fatalf("unexpected error downgrading server: %s", err)
 	}
 
-	_, err = v5server.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := v5server.(tfprotov5.FunctionServer)
+
+	if !ok {
+		t.Fatal("v5server should implement tfprotov5.FunctionServer")
+	}
+
+	// _, err = v5server.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
+	_, err = functionServer.CallFunction(ctx, &tfprotov5.CallFunctionRequest{
 		Name: "test_function",
 	})
 
@@ -336,7 +344,15 @@ func TestV5ToV6ServerGetFunctions(t *testing.T) {
 		t.Fatalf("unexpected error downgrading server: %s", err)
 	}
 
-	_, err = v5server.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
+	// Reference: https://github.com/hashicorp/terraform-plugin-mux/issues/210
+	functionServer, ok := v5server.(tfprotov5.FunctionServer)
+
+	if !ok {
+		t.Fatal("v5server should implement tfprotov5.FunctionServer")
+	}
+
+	//_, err = v5server.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
+	_, err = functionServer.GetFunctions(ctx, &tfprotov5.GetFunctionsRequest{})
 
 	if err != nil {
 		t.Fatalf("unexpected error: %s", err)