diff --git a/runtime/marshal_json_test.go b/runtime/marshal_json_test.go
index 6ab9699e48c..94280990be2 100644
--- a/runtime/marshal_json_test.go
+++ b/runtime/marshal_json_test.go
@@ -204,6 +204,13 @@ var (
 		{data: (*string)(nil), json: "null"},
 		{data: new(empty.Empty), json: "{}"},
 		{data: examplepb.NumericEnum_ONE, json: "1"},
+		{data: nil, json: "null"},
+		{data: (*string)(nil), json: "null"},
+		{data: []interface{}{nil, "foo", -1, 1.234, true}, json: `[null,"foo",-1,1.234,true]`},
+		{
+			data: map[string]interface{}{"bar": nil, "baz": -1, "fiz": 1.234, "foo": true},
+			json: `{"bar":null,"baz":-1,"fiz":1.234,"foo":true}`,
+		},
 		{
 			data: (*examplepb.NumericEnum)(proto.Int32(int32(examplepb.NumericEnum_ONE))),
 			json: "1",
diff --git a/runtime/marshal_jsonpb.go b/runtime/marshal_jsonpb.go
index 0a0d130bba6..2a90a4403cf 100644
--- a/runtime/marshal_jsonpb.go
+++ b/runtime/marshal_jsonpb.go
@@ -21,9 +21,7 @@ func (*JSONPb) ContentType() string {
 	return "application/json"
 }
 
-// Marshal marshals "v" into JSON
-// Currently it can marshal only proto.Message.
-// TODO(yugui) Support fields of primitive types in a message.
+// Marshal marshals "v" into JSON.
 func (j *JSONPb) Marshal(v interface{}) ([]byte, error) {
 	if _, ok := v.(proto.Message); !ok {
 		return j.marshalNonProtoField(v)
@@ -55,6 +53,9 @@ func (j *JSONPb) marshalTo(w io.Writer, v interface{}) error {
 // i.e. primitive types, enums; pointers to primitives or enums; maps from
 // integer/string types to primitives/enums/pointers to messages.
 func (j *JSONPb) marshalNonProtoField(v interface{}) ([]byte, error) {
+	if v == nil {
+		return []byte("null"), nil
+	}
 	rv := reflect.ValueOf(v)
 	for rv.Kind() == reflect.Ptr {
 		if rv.IsNil() {
diff --git a/runtime/marshal_jsonpb_test.go b/runtime/marshal_jsonpb_test.go
index 679283b4511..0c3d01bfeba 100644
--- a/runtime/marshal_jsonpb_test.go
+++ b/runtime/marshal_jsonpb_test.go
@@ -121,25 +121,23 @@ func TestJSONPbMarshal(t *testing.T) {
 
 func TestJSONPbMarshalFields(t *testing.T) {
 	var m runtime.JSONPb
-	for _, spec := range []struct {
-		val  interface{}
-		want string
-	}{} {
-		buf, err := m.Marshal(spec.val)
+	m.EnumsAsInts = true // builtin fixtures include an enum, expected to be marshaled as int
+	for _, spec := range builtinFieldFixtures {
+		buf, err := m.Marshal(spec.data)
 		if err != nil {
-			t.Errorf("m.Marshal(%#v) failed with %v; want success", spec.val, err)
+			t.Errorf("m.Marshal(%#v) failed with %v; want success", spec.data, err)
 		}
-		if got, want := string(buf), spec.want; got != want {
-			t.Errorf("m.Marshal(%#v) = %q; want %q", spec.val, got, want)
+		if got, want := string(buf), spec.json; got != want {
+			t.Errorf("m.Marshal(%#v) = %q; want %q", spec.data, got, want)
 		}
 	}
 
-	m.EnumsAsInts = true
+	m.EnumsAsInts = false
 	buf, err := m.Marshal(examplepb.NumericEnum_ONE)
 	if err != nil {
 		t.Errorf("m.Marshal(%#v) failed with %v; want success", examplepb.NumericEnum_ONE, err)
 	}
-	if got, want := string(buf), "1"; got != want {
+	if got, want := string(buf), `"ONE"`; got != want {
 		t.Errorf("m.Marshal(%#v) = %q; want %q", examplepb.NumericEnum_ONE, got, want)
 	}
 }