diff --git a/mozilla/layout/tables/nsTablePainter.cpp b/mozilla/layout/tables/nsTablePainter.cpp
index ee6128188e4..4a3443fa2fa 100644
--- a/mozilla/layout/tables/nsTablePainter.cpp
+++ b/mozilla/layout/tables/nsTablePainter.cpp
@@ -587,6 +587,9 @@ TableBackgroundPainter::PaintCell(nsTableCellFrame* aCell,
 
   PRInt32 colIndex;
   aCell->GetColIndex(colIndex);
+  NS_ASSERTION(colIndex < mNumCols, "prevent array boundary violation");
+  if (mNumCols <= colIndex)
+    return NS_OK;
 
   //Paint column group background
   if (mCols && mCols[colIndex].mColGroup && mCols[colIndex].mColGroup->IsVisible()) {