diff --git a/dom/messagechannel/MessagePort.cpp b/dom/messagechannel/MessagePort.cpp
index 3b8b5e633254..85e9e5114218 100644
--- a/dom/messagechannel/MessagePort.cpp
+++ b/dom/messagechannel/MessagePort.cpp
@@ -900,7 +900,9 @@ MessagePort::RemoveDocFromBFCache()
   }
 
   nsPIDOMWindow* window = GetOwner();
-  MOZ_ASSERT(window);
+  if (!window) {
+    return;
+  }
 
   nsIDocument* doc = window->GetExtantDoc();
   if (!doc) {