fix the nil judge

@@ -131,7 +131,7 @@ func getFieldType(val reflect.Value) (ft int, err error) {
 	case reflect.String:
 		ft = TypeCharField
 	default:
-		if elm.IsNil() {
+		if elm.Interface() == nil {
 			panic(fmt.Errorf("%s is nil pointer, may be miss setting tag", val))
 		}
 		switch elm.Interface().(type) {