diff --git UserScriptLoader/UserScriptLoader.uc.js UserScriptLoader/UserScriptLoader.uc.js
index 16a23a6..c070e2c 100644
--- UserScriptLoader/UserScriptLoader.uc.js
+++ UserScriptLoader/UserScriptLoader.uc.js
@@ -973,7 +973,7 @@ USL.injectScripts = function(safeWindow, rsflag) {
            return;
        }

-       let sandbox = new Cu.Sandbox(safeWindow);
+       let sandbox = new Cu.Sandbox(safeWindow, {sandboxPrototype: safeWindow});
        let GM_API = new USL.API(script, sandbox, safeWindow, aDocument);
        for (let n in GM_API)
            sandbox[n] = GM_API[n];