--- a/testing/marionette/error.js
+++ b/testing/marionette/error.js
@@ -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/. */
 
 "use strict";
 
 const {interfaces: Ci, utils: Cu} = Components;
 
-const errors = [
+const ERRORS = [
   "ElementNotAccessibleError",
   "ElementNotVisibleError",
   "InvalidArgumentError",
   "InvalidElementStateError",
   "InvalidSelectorError",
   "InvalidSessionIdError",
   "JavaScriptError",
   "NoAlertOpenError",
@@ -24,20 +24,31 @@ const errors = [
   "TimeoutError",
   "UnableToSetCookieError",
   "UnknownCommandError",
   "UnknownError",
   "UnsupportedOperationError",
   "WebDriverError",
 ];
 
-this.EXPORTED_SYMBOLS = ["error"].concat(errors);
+this.EXPORTED_SYMBOLS = ["error"].concat(ERRORS);
 
 this.error = {};
 
+error.BuiltinErrors = {
+  Error: 0,
+  EvalError: 1,
+  InternalError: 2,
+  RangeError: 3,
+  ReferenceError: 4,
+  SyntaxError: 5,
+  TypeError: 6,
+  URIError: 7,
+};
+
 /**
  * Checks if obj is an instance of the Error prototype in a safe manner.
  * Prefer using this over using instanceof since the Error prototype
  * isn't unique across browsers, and XPCOM nsIException's are special
  * snowflakes.
  *
  * @param {*} val
  *     Any value that should be undergo the test for errorness.
@@ -45,26 +56,26 @@ this.error = {};
  *     True if error, false otherwise.
  */
 error.isError = function(val) {
   if (val === null || typeof val != "object") {
     return false;
   } else if (val instanceof Ci.nsIException) {
     return true;
   } else {
-    return Object.getPrototypeOf(val) == "Error";
+    return Object.getPrototypeOf(val) in error.BuiltinErrors;
   }
 };
 
 /**
  * Checks if obj is an object in the WebDriverError prototypal chain.
  */
 error.isWebDriverError = function(obj) {
   return error.isError(obj) &&
-      ("name" in obj && errors.indexOf(obj.name) >= 0);
+      ("name" in obj && ERRORS.indexOf(obj.name) >= 0);
 };
 
 /**
  * Unhandled error reporter.  Dumps the error and its stacktrace to console,
  * and reports error to the Browser Console.
  */
 error.report = function(err) {
   let msg = `Marionette threw an error: ${error.stringify(err)}`;
@@ -323,14 +334,14 @@ this.UnsupportedOperationError = functio
   WebDriverError.call(this, msg);
   this.name = "UnsupportedOperationError";
   this.status = "unsupported operation";
 };
 UnsupportedOperationError.prototype = Object.create(WebDriverError.prototype);
 
 const nameLookup = new Map();
 const statusLookup = new Map();
-for (let s of errors) {
+for (let s of ERRORS) {
   let cls = this[s];
   let inst = new cls();
   nameLookup.set(inst.name, cls);
   statusLookup.set(inst.status, cls);
 };

--- a/testing/marionette/test_error.js
+++ b/testing/marionette/test_error.js
@@ -9,32 +9,59 @@ Cu.import("chrome://marionette/content/e
 function run_test() {
   run_next_test();
 }
 
 function notok(condition) {
   ok(!(condition));
 }
 
+add_test(function test_BuiltinErrors() {
+  ok("Error" in error.BuiltinErrors);
+  ok("EvalError" in error.BuiltinErrors);
+  ok("InternalError" in error.BuiltinErrors);
+  ok("RangeError" in error.BuiltinErrors);
+  ok("ReferenceError" in error.BuiltinErrors);
+  ok("SyntaxError" in error.BuiltinErrors);
+  ok("TypeError" in error.BuiltinErrors);
+  ok("URIError" in error.BuiltinErrors);
+
+  run_next_test();
+});
+
 add_test(function test_isError() {
   notok(error.isError(null));
   notok(error.isError([]));
   notok(error.isError(new Date()));
 
   ok(error.isError(new Components.Exception()));
   ok(error.isError(new Error()));
+  ok(error.isError(new EvalError()));
+  ok(error.isError(new InternalError()));
+  ok(error.isError(new RangeError()));
+  ok(error.isError(new ReferenceError()));
+  ok(error.isError(new SyntaxError()));
+  ok(error.isError(new TypeError()));
+  ok(error.isError(new URIError()));
   ok(error.isError(new WebDriverError()));
   ok(error.isError(new InvalidArgumentError()));
 
   run_next_test();
 });
 
 add_test(function test_isWebDriverError() {
   notok(error.isWebDriverError(new Components.Exception()));
   notok(error.isWebDriverError(new Error()));
+  notok(error.isWebDriverError(new EvalError()));
+  notok(error.isWebDriverError(new InternalError()));
+  notok(error.isWebDriverError(new RangeError()));
+  notok(error.isWebDriverError(new ReferenceError()));
+  notok(error.isWebDriverError(new SyntaxError()));
+  notok(error.isWebDriverError(new TypeError()));
+  notok(error.isWebDriverError(new URIError()));
 
   ok(error.isWebDriverError(new WebDriverError()));
   ok(error.isWebDriverError(new InvalidArgumentError()));
 
   run_next_test();
 });
 
 add_test(function test_stringify() {