diff --git a/tests/test_double_serializer.c b/tests/test_double_serializer.c
index 38de6b5..8b2487a 100644
--- a/tests/test_double_serializer.c
+++ b/tests/test_double_serializer.c
@@ -20,7 +20,7 @@ int main()
 	printf("obj.to_string(standard)=%s\n", json_object_to_json_string(obj));
 
 	printf("Test default serializer with custom userdata:\n");
-	((struct json_object *)obj)->_userdata = udata;
+	obj->_userdata = udata;
 	printf("obj.to_string(userdata)=%s\n", json_object_to_json_string(obj));
 
 	printf("Test explicit serializer with custom userdata:\n");