--- a/toolkit/content/process-content.js
+++ b/toolkit/content/process-content.js
@@ -47,17 +47,17 @@ let ProcessObserver = {
       case "content-document-global-created": {
 
         // Strip the hash from the URL, because it's not part of the origin.
         let window = subject;
         let url = window.document.documentURI.replace(/[\#|\?].*$/, "");
 
         let registeredURLs = Services.cpmm.sharedData.get("RemotePageManager:urls");
 
-        if (!registeredURLs.has(url))
+        if (!registeredURLs || !registeredURLs.has(url))
           return;
 
         // Get the frame message manager for this window so we can associate this
         // page with a browser element
         let messageManager = window.QueryInterface(Ci.nsIInterfaceRequestor)
                                    .getInterface(Ci.nsIDocShell)
                                    .QueryInterface(Ci.nsIInterfaceRequestor)
                                    .getInterface(Ci.nsIContentFrameMessageManager);