diff --git a/tests/testgtk.c b/tests/testgtk.c
index b704c6f20e..e21861421a 100644
--- a/tests/testgtk.c
+++ b/tests/testgtk.c
@@ -2426,6 +2426,8 @@ grippy_draw (GtkWidget *area, cairo_t *cr, GdkWindowEdge edge)
     case GDK_WINDOW_EDGE_SOUTH_EAST:
       sides = GTK_JUNCTION_CORNER_BOTTOMRIGHT;
       break;
+    default:
+      g_assert_not_reached();
     }
 
   context = gtk_widget_get_style_context (area);