diff --git a/gdk/x11/gdkglcontext-x11.c b/gdk/x11/gdkglcontext-x11.c
index 72c775885b..834d23ac93 100644
--- a/gdk/x11/gdkglcontext-x11.c
+++ b/gdk/x11/gdkglcontext-x11.c
@@ -648,7 +648,7 @@ on_gl_surface_xevent (GdkGLContext   *context,
           case GL_TIMEOUT_EXPIRED:
             break;
           default:
-            g_assert_not_reached ();
+            g_error ("glClientWaitSync returned unexpected result: %x", (guint) wait_result);
         }
     }