diff --git a/test-fixtures/test-utils.c b/test-fixtures/test-utils.c
index 5980eeb54..b85efcded 100644
--- a/test-fixtures/test-utils.c
+++ b/test-fixtures/test-utils.c
@@ -145,6 +145,9 @@ test_utils_init (TestFlags requirement_flags,
       is_boolean_env_set ("V"))
     cogl_test_is_verbose = TRUE;
 
+  /* NB: This doesn't have any effect since commit 47444dac of glib
+   * because the environment variable is read in a magic constructor
+   * so it is too late to set them here */
   if (g_getenv ("G_DEBUG"))
     {
       char *debug = g_strconcat (g_getenv ("G_DEBUG"), ",fatal-warnings", NULL);
diff --git a/tests/run-tests.sh b/tests/run-tests.sh
index a00494338..388fa036c 100755
--- a/tests/run-tests.sh
+++ b/tests/run-tests.sh
@@ -1,5 +1,13 @@
 #!/bin/bash
 
+if test -z "$G_DEBUG"; then
+    G_DEBUG=fatal-warnings
+else
+    G_DEBUG="$G_DEBUG,fatal-warnings"
+fi
+
+export G_DEBUG
+
 ENVIRONMENT_CONFIG=$1
 shift