diff --git a/widget/src/mac/nsMacEventHandler.cpp b/widget/src/mac/nsMacEventHandler.cpp
index 2c7bb6600171..ab3e1fb7fb6e 100644
--- a/widget/src/mac/nsMacEventHandler.cpp
+++ b/widget/src/mac/nsMacEventHandler.cpp
@@ -146,6 +146,10 @@ static nsEventStatus
 HandleScrollEvent ( EventMouseWheelAxis inAxis, PRBool inByLine, PRInt32 inDelta,
                      Point inMouseLoc, nsIWidget* inWidget )
 {
+  NS_ASSERTION(inWidget, "HandleScrollEvent doesn't work with a null widget");
+  if (!inWidget)
+    return nsEventStatus_eIgnore;
+
   nsMouseScrollEvent scrollEvent;
   
   scrollEvent.scrollFlags =