diff --git a/gobject/tests/type-flags.c b/gobject/tests/type-flags.c
index c4813bf3f..e4e501850 100644
--- a/gobject/tests/type-flags.c
+++ b/gobject/tests/type-flags.c
@@ -141,6 +141,8 @@ main (int argc, char *argv[])
 {
   g_test_init (&argc, &argv, NULL);
 
+  g_setenv ("G_ENABLE_DIAGNOSTIC", "1", TRUE);
+
   g_test_add_func ("/type/flags/final", test_type_flags_final);
   g_test_add_func ("/type/flags/deprecated", test_type_flags_deprecated);