diff --git a/org-ol-tree.el b/org-ol-tree.el
index 9d8420f..05718ff 100644
--- a/org-ol-tree.el
+++ b/org-ol-tree.el
@@ -326,7 +326,7 @@ The outline chooses the theme based on the following criteria:
 
 To find out if Emacs is running in GUI mode, we query the variable
 `window-system'."
-  (member window-system '(x w32 ns)))
+  (memq window-system '(pgtk x w32 ns)))