diff --git a/netwerk/protocol/about/src/nsAboutProtocolHandler.cpp b/netwerk/protocol/about/src/nsAboutProtocolHandler.cpp
index 7263e9f654c7..02e4a7e268f9 100644
--- a/netwerk/protocol/about/src/nsAboutProtocolHandler.cpp
+++ b/netwerk/protocol/about/src/nsAboutProtocolHandler.cpp
@@ -47,6 +47,7 @@
 #include "nsReadableUtils.h"
 #include "nsNetCID.h"
 #include "nsAboutProtocolUtils.h"
+#include "nsNetError.h"
 
 static NS_DEFINE_CID(kSimpleURICID,     NS_SIMPLEURI_CID);
 
@@ -152,6 +153,12 @@ nsAboutProtocolHandler::NewChannel(nsIURI* uri, nsIChannel* *result)
 
     // mumble...
 
+    if (rv == NS_ERROR_FACTORY_NOT_REGISTERED) {
+        // This looks like an about: we don't know about.  Convert
+        // this to an invalid URI error.
+        rv = NS_ERROR_MALFORMED_URI;
+    }
+
     return rv;
 }