diff --git a/mozilla/layout/base/nsCSSFrameConstructor.cpp b/mozilla/layout/base/nsCSSFrameConstructor.cpp
index d4d1b907c90..980cbdfb137 100644
--- a/mozilla/layout/base/nsCSSFrameConstructor.cpp
+++ b/mozilla/layout/base/nsCSSFrameConstructor.cpp
@@ -1808,7 +1808,9 @@ nsresult
 nsMathMLmtableCreator::CreateTableCellFrame(nsIFrame*  aParentFrame,
                                             nsIFrame** aNewFrame)
 {
-  NS_ASSERTION(!IsBorderCollapse(aParentFrame), "not implemented");
+  if (IsBorderCollapse(aParentFrame))
+    return NS_NewTableCellFrame(mPresShell, PR_TRUE, aNewFrame);
+
   return NS_NewMathMLmtdFrame(mPresShell, aNewFrame);
 }