diff --git a/ChangeLog b/ChangeLog
index ee20a5627..6730f3d37 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2006-12-12  Thomas Thurman  <thomas@thurman.org.uk>
+
+        * src/compositor.c (do_effect): Sanity check to avoid dereferencing
+	a null pointer.
+
 2006-12-10  Thomas Thurman  <thomas@thurman.org.uk>
 
         * configure.in: post-release bump to 2.17.5.
diff --git a/src/compositor.c b/src/compositor.c
index 05b4e1fb2..1b8ed6b0c 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -121,6 +121,15 @@ do_effect (MetaEffect *effect,
     MetaCompWindow *window;
 
     screen = meta_comp_screen_get_by_xwindow (get_xid (effect->window));
+
+    if (!screen)
+      {
+        /* sanity check: if no screen is found, bail */
+        meta_warning ("No screen found for %s (%ld); aborting effect.\n",
+            effect->window->desc, get_xid (effect->window));
+        return;
+      }
+
     window = meta_comp_screen_lookup_window (screen, get_xid (effect->window));
     
     switch (effect->type)