--- a/testing/marionette/evaluate.js
+++ b/testing/marionette/evaluate.js
@@ -98,17 +98,17 @@ this.evaluate = {};
  * @throws ScriptTimeoutError
  *   If the script was interrupted due to script timeout.
  */
 evaluate.sandbox = function (sb, script, args = [], opts = {}) {
   let scriptTimeoutID, timeoutHandler, unloadHandler;
 
   let promise = new Promise((resolve, reject) => {
     let src = "";
-    sb[COMPLETE] = resolve;
+    sb[COMPLETE] = sandbox.cloneInto(resolve, sb);
     timeoutHandler = () => reject(new ScriptTimeoutError("Timed out"));
     unloadHandler = sandbox.cloneInto(
         () => reject(new JavaScriptError("Document was unloaded during execution")),
         sb);
 
     // wrap in function
     if (!opts.directInject) {
       if (opts.async) {