diff --git a/mozilla/layout/generic/nsLineBox.cpp b/mozilla/layout/generic/nsLineBox.cpp
index 6fe6413a15b..3dd6910c33e 100644
--- a/mozilla/layout/generic/nsLineBox.cpp
+++ b/mozilla/layout/generic/nsLineBox.cpp
@@ -388,6 +388,8 @@ nsLineBox::RemoveFloater(nsIFrame* aFrame)
 void
 nsLineBox::SetCombinedArea(const nsRect& aCombinedArea)
 {
+  NS_ASSERTION(aCombinedArea.width >= 0, "illegal width for combined area");
+  NS_ASSERTION(aCombinedArea.height >= 0, "illegal height for combined area");
   if (aCombinedArea != mBounds) {
     if (mData) {
       mData->mCombinedArea = aCombinedArea;
diff --git a/mozilla/layout/html/base/src/nsLineBox.cpp b/mozilla/layout/html/base/src/nsLineBox.cpp
index 6fe6413a15b..3dd6910c33e 100644
--- a/mozilla/layout/html/base/src/nsLineBox.cpp
+++ b/mozilla/layout/html/base/src/nsLineBox.cpp
@@ -388,6 +388,8 @@ nsLineBox::RemoveFloater(nsIFrame* aFrame)
 void
 nsLineBox::SetCombinedArea(const nsRect& aCombinedArea)
 {
+  NS_ASSERTION(aCombinedArea.width >= 0, "illegal width for combined area");
+  NS_ASSERTION(aCombinedArea.height >= 0, "illegal height for combined area");
   if (aCombinedArea != mBounds) {
     if (mData) {
       mData->mCombinedArea = aCombinedArea;