--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -1267,16 +1267,20 @@ nsFrameLoader::SwapWithOtherRemoteLoader
   }
 
   nsIPresShell* ourShell = ourDoc->GetShell();
   nsIPresShell* otherShell = otherDoc->GetShell();
   if (!ourShell || !otherShell) {
     return NS_ERROR_NOT_IMPLEMENTED;
   }
 
+  if (!mRemoteBrowser || !aOther->mRemoteBrowser) {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
   if (mRemoteBrowser->IsIsolatedMozBrowserElement() !=
       aOther->mRemoteBrowser->IsIsolatedMozBrowserElement()) {
     return NS_ERROR_NOT_IMPLEMENTED;
   }
 
   // When we swap docShells, maybe we have to deal with a new page created just
   // for this operation. In this case, the browser code should already have set
   // the correct userContextId attribute value in the owning XULElement, but our