diff --git a/engine/builtin_test.go b/engine/builtin_test.go
index 3d818bc..670db66 100644
--- a/engine/builtin_test.go
+++ b/engine/builtin_test.go
@@ -37,6 +37,12 @@ func TestCall(t *testing.T) {
 	vm.Register0(NewAtom("do_not_call_wrapped"), func(*VM, Cont, *Env) *Promise {
 		panic(errors.New("told you"))
 	})
+	vm.Register0(NewAtom("do_not_call_exception"), func(*VM, Cont, *Env) *Promise {
+		panic(Exception{NewAtom("error").Apply(NewAtom("panic_error"), NewAtom("told you"))})
+	})
+	vm.Register0(NewAtom("do_not_call_misc_error"), func(*VM, Cont, *Env) *Promise {
+		panic(42)
+	})
 	assert.NoError(t, vm.Compile(context.Background(), `
 foo.
 foo(_, _).
@@ -64,9 +70,11 @@ f(g([a, [b, c|X], Y{x:5}])).
 
 		{title: `cover all`, goal: atomComma.Apply(atomCut, NewAtom("f").Apply(NewAtom("g").Apply(List(NewAtom("a"), PartialList(NewVariable(), NewAtom("b"), NewAtom("c")), makeDict(NewAtom("foo"), NewAtom("x"), Integer(5)))))), ok: true},
 		{title: `out of memory`, goal: NewAtom("foo").Apply(NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable()), err: resourceError(resourceMemory, nil), mem: 1},
-		{title: `panic`, goal: NewAtom("do_not_call"), err: PanicError{errors.New("told you")}},
-		{title: `panic (lazy)`, goal: NewAtom("lazy_do_not_call"), err: PanicError{errors.New("told you")}},
-		{title: `panic (wrapped)`, goal: NewAtom("do_not_call_wrapped"), err: PanicError{errors.New("told you")}},
+		{title: `panic`, goal: NewAtom("do_not_call"), err: Exception{NewAtom("error").Apply(NewAtom("panic_error").Apply(NewAtom("told you")))}},
+		{title: `panic (lazy)`, goal: NewAtom("lazy_do_not_call"), err: Exception{NewAtom("error").Apply(NewAtom("panic_error").Apply(NewAtom("told you")))}},
+		{title: `panic (wrapped)`, goal: NewAtom("do_not_call_wrapped"), err: Exception{NewAtom("error").Apply(NewAtom("panic_error").Apply(NewAtom("told you")))}},
+		{title: `panic (exception)`, goal: NewAtom("do_not_call_exception"), err: Exception{NewAtom("error").Apply(NewAtom("panic_error").Apply(NewAtom("told you")))}},
+		{title: `panic (misc)`, goal: NewAtom("do_not_call_misc_error"), err: Exception{NewAtom("error").Apply(NewAtom("panic_error").Apply(NewAtom("42")))}},
 	}
 
 	for _, tt := range tests {
diff --git a/engine/promise.go b/engine/promise.go
index b92774f..7a4fd0d 100644
--- a/engine/promise.go
+++ b/engine/promise.go
@@ -162,10 +162,12 @@ func ensurePromise(p **Promise) {
 
 func panicError(r interface{}) error {
 	switch r := r.(type) {
+	case Exception:
+		return r
 	case error:
-		return PanicError{r}
+		return Exception{term: atomError.Apply(NewAtom("panic_error").Apply(NewAtom(r.Error())))}
 	default:
-		return PanicError{fmt.Errorf("%v", r)}
+		return Exception{term: atomError.Apply(NewAtom("panic_error").Apply(NewAtom(fmt.Sprintf("%v", r))))}
 	}
 }