diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index dd4e597e39f..2e15227cdac 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -8304,7 +8304,10 @@ nsCSSFrameConstructor::EndUpdate()
     RecalcQuotesAndCounters();
     NS_ASSERTION(mUpdateCount == 1, "Odd update count");
   }
-  --mUpdateCount;
+  // Negative update counts don't make sense
+  if (mUpdateCount > 0) {
+    --mUpdateCount;
+  }
 }
 
 void