--- a/layout/generic/TextOverflow.cpp
+++ b/layout/generic/TextOverflow.cpp
@@ -174,17 +174,17 @@ InflateRight(nsRect* aRect, bool aInfini
   }
 }
 
 static bool
 IsFrameDescendantOfAny(nsIFrame* aChild,
                        const TextOverflow::FrameHashtable& aSetOfFrames,
                        nsIFrame* aCommonAncestor)
 {
-  for (nsIFrame* f = aChild; f != aCommonAncestor;
+  for (nsIFrame* f = aChild; f && f != aCommonAncestor;
        f = nsLayoutUtils::GetCrossDocParentFrame(f)) {
     if (aSetOfFrames.GetEntry(f)) {
       return true;
     }
   }
   return false;
 }