diff --git a/layout/xul/base/src/nsBox.h b/layout/xul/base/src/nsBox.h
index 4165148631f..6bf9d0c9f40 100644
--- a/layout/xul/base/src/nsBox.h
+++ b/layout/xul/base/src/nsBox.h
@@ -172,7 +172,7 @@ private:
   //nscoord mY;
 };
 
-#ifdef DEBUG
+#ifdef DEBUG_LAYOUT
 #define NS_BOX_ASSERTION(box,expr,str) \
   if (!(expr)) { \
        box->DumpBox(stdout); \