diff --git a/widget/src/windows/nsWindow.cpp b/widget/src/windows/nsWindow.cpp
index 481b8fb8f279..bdb587899fcd 100644
--- a/widget/src/windows/nsWindow.cpp
+++ b/widget/src/windows/nsWindow.cpp
@@ -644,7 +644,7 @@ nsWindow::StandardWindowCreate(nsIWidget *aParent,
       parent = NULL;
   } else if (mWindowType == eWindowType_invisible) {
     // Make sure CreateWindowEx succeeds at creating a toplevel window
-    style &= ~WS_CHILDWINDOW;
+    style &= ~0x40000000; // WS_CHILDWINDOW
   } else if (nsnull != aInitData) {
     // See if the caller wants to explictly set clip children and clip siblings
     if (aInitData->clipChildren) {