diff --git a/mozilla/netwerk/base/src/nsProxyAutoConfig.js b/mozilla/netwerk/base/src/nsProxyAutoConfig.js
index c56cfb7a650..c2a55fffd4d 100644
--- a/mozilla/netwerk/base/src/nsProxyAutoConfig.js
+++ b/mozilla/netwerk/base/src/nsProxyAutoConfig.js
@@ -96,7 +96,7 @@ nsProxyAutoConfig.prototype = {
             return null;
 
         // Call the original function
-        return this._findProxyForURL(testURI, testHost);
+        return this._findProxyForURL.call(this._sandBox, testURI, testHost);
     }
 }