diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index e66e6a8d8d3f..7a313bc0c1ac 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -677,7 +677,7 @@ nsHTMLScrollFrame::GetPrefWidth(nsIRenderingContext *aRenderingContext)
     nsSize vScrollbarPrefSize(0, 0);
     GetScrollbarMetrics(bls, mInner.mVScrollbarBox,
                         nsnull, &vScrollbarPrefSize, PR_TRUE);
-    result += vScrollbarPrefSize.width;
+    result = NSCoordSaturatingAdd(result, vScrollbarPrefSize.width);
   }
 
   return result;