--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -866,36 +866,34 @@ GeckoDriver.prototype.executeAsyncScript
   resp.body.value = yield this.execute_(script, args, scriptTimeout, opts);
 };
 
 GeckoDriver.prototype.execute_ = function (script, args, timeout, opts = {}) {
   switch (this.context) {
     case Context.CONTENT:
       // evaluate in content with lasting side-effects
       if (!opts.sandboxName) {
-        return this.listener.execute(script, args, timeout, opts)
-            .then(evaluate.toJSON);
+        return this.listener.execute(script, args, timeout, opts);
 
       // evaluate in content with sandbox
       } else {
-        return this.listener.executeInSandbox(script, args, timeout, opts)
-            .then(evaluate.toJSON);
+        return this.listener.executeInSandbox(script, args, timeout, opts);
       }
 
     case Context.CHROME:
       let sb = this.sandboxes.get(opts.sandboxName, opts.newSandbox);
       if (opts.sandboxName) {
         sb = sandbox.augment(sb, new logging.Adapter(this.marionetteLog));
         sb = sandbox.augment(sb, {global: sb});
       }
 
       opts.timeout = timeout;
       let wargs = evaluate.fromJSON(args, this.curBrowser.seenEls, sb.window);
-      return evaluate.sandbox(sb, script, wargs, opts)
-          .then(res => evaluate.toJSON(res, this.curBrowser.seenEls));
+      let evaluatePromise = evaluate.sandbox(sb, script, wargs, opts);
+      return evaluatePromise.then(res => evaluate.toJSON(res, this.curBrowser.seenEls));
   }
 };
 
 /**
  * Navigate to given URL.
  *
  * Navigates the current browsing context to the given URL and waits for
  * the document to load or the session's page timeout duration to elapse

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py
@@ -1,17 +1,17 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import os
 import urllib
 
 from marionette_driver import By, errors
-from marionette_driver.marionette import Alert, HTMLElement
+from marionette_driver.marionette import HTMLElement
 from marionette_driver.wait import Wait
 
 from marionette_harness import MarionetteTestCase, skip_if_mobile, WindowManagerMixin
 
 
 def inline(doc):
     return "data:text/html;charset=utf-8,{}".format(urllib.quote(doc))
 
@@ -26,33 +26,16 @@ globals = set([
               "navigator",
               "URL",
               "window",
               ])
 
 
 class TestExecuteContent(MarionetteTestCase):
 
-    def alert_present(self):
-        try:
-            Alert(self.marionette).text
-            return True
-        except errors.NoAlertPresentException:
-            return False
-
-    def wait_for_alert_closed(self, timeout=None):
-        Wait(self.marionette, timeout=timeout).until(
-            lambda _: not self.alert_present())
-
-    def tearDown(self):
-        if self.alert_present():
-            alert = self.marionette.switch_to_alert()
-            alert.dismiss()
-            self.wait_for_alert_closed()
-
     def assert_is_defined(self, property, sandbox="default"):
         self.assertTrue(self.marionette.execute_script(
             "return typeof arguments[0] != 'undefined'", [property], sandbox=sandbox),
             "property {} is undefined".format(property))
 
     def assert_is_web_element(self, element):
         self.assertIsInstance(element, HTMLElement)
 
@@ -352,21 +335,16 @@ class TestExecuteContent(MarionetteTestC
             return {
               toJSON () {
                 return document.documentElement;
               }
             }""",
             sandbox=None)
         self.assert_is_web_element(el)
 
-    def test_return_value_on_alert(self):
-        res = self.marionette._send_message("executeScript", {"script": "alert()"})
-        self.assertIn("value", res)
-        self.assertIsNone(res)
-
 
 class TestExecuteChrome(WindowManagerMixin, TestExecuteContent):
 
     def setUp(self):
         super(TestExecuteChrome, self).setUp()
 
         self.marionette.set_context("chrome")
 
@@ -425,19 +403,16 @@ class TestExecuteChrome(WindowManagerMix
         pass
 
     def test_system_sandbox_wrappedjsobject(self):
         pass
 
     def test_access_chrome_objects_in_event_listeners(self):
         pass
 
-    def test_return_value_on_alert(self):
-        pass
-
 
 class TestElementCollections(MarionetteTestCase):
 
     def assertSequenceIsInstance(self, seq, typ):
         for item in seq:
             self.assertIsInstance(item, typ)
 
     def test_array(self):