fix: improve the code

function.go

@@ -34,11 +34,9 @@ func reflectFunc(fn any) *reflectedFn {
 	case 1:
 		if t.Out(0).Implements(errorType) {
 			rFn.outFn = func(out []reflect.Value) (interface{}, error) {
-				if err, ok := out[0].Interface().(error); ok {
-					return nil, err
-				}
+				err, _ := out[0].Interface().(error)
 
-				return nil, nil
+				return nil, err
 			}
 		} else {
 			rFn.outFn = func(out []reflect.Value) (interface{}, error) {

handler_test.go

@@ -145,6 +145,19 @@ func TestHandler(t *testing.T) {
 			expectedResponse: []byte(""),
 			expectedStatus:   http.StatusNoContent,
 		},
+		{
+			name: "double nil error response",
+			req: func() *http.Request {
+				return httptest.NewRequest(http.MethodGet, "https://example.com", mustOpen(t, "valid.json"))
+			}(),
+			handler: func() (map[string]interface{}, error) {
+				return map[string]interface{}{
+					"msg": "hello",
+				}, nil
+			},
+			expectedResponse: []byte(`{"msg":"hello"}` + "\n"),
+			expectedStatus:   http.StatusOK,
+		},
 		{
 			name: "panic response",
 			req: func() *http.Request {