diff --git a/src/log/slog/json_handler.go b/src/log/slog/json_handler.go
index 96545d58d6..ce249acfd3 100644
--- a/src/log/slog/json_handler.go
+++ b/src/log/slog/json_handler.go
@@ -135,7 +135,8 @@ func appendJSONValue(s *handleState, v Value) error {
 		s.appendTime(v.Time())
 	case KindAny:
 		a := v.Any()
-		if err, ok := a.(error); ok {
+		_, jm := a.(json.Marshaler)
+		if err, ok := a.(error); ok && !jm {
 			s.appendString(err.Error())
 		} else {
 			return appendJSONMarshal(s.buf, a)
diff --git a/src/log/slog/json_handler_test.go b/src/log/slog/json_handler_test.go
index 0a38969f46..7c683f0d34 100644
--- a/src/log/slog/json_handler_test.go
+++ b/src/log/slog/json_handler_test.go
@@ -67,6 +67,12 @@ func (j jsonMarshaler) MarshalJSON() ([]byte, error) {
 	return []byte(fmt.Sprintf(`[%q]`, j.s)), nil
 }
 
+type jsonMarshalerError struct {
+	jsonMarshaler
+}
+
+func (jsonMarshalerError) Error() string { return "oops" }
+
 func TestAppendJSONValue(t *testing.T) {
 	// On most values, jsonAppendAttrValue should agree with json.Marshal.
 	for _, value := range []any{
@@ -82,6 +88,7 @@ func TestAppendJSONValue(t *testing.T) {
 		time.Minute,
 		testTime,
 		jsonMarshaler{"xyz"},
+		jsonMarshalerError{jsonMarshaler{"pqr"}},
 	} {
 		got := jsonValueString(t, AnyValue(value))
 		want, err := marshalJSON(value)