--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -968,17 +968,19 @@ element.scrollIntoView = function(el) {
  * @param {*} node
  *     Element must have a <code>nodeType</code> property and its
  *     value must be <code>Element.prototype.ELEMENT_NODE</code>.
  *
  * @return {boolean}
  *     True if <var>node</var> is a DOM element, false otherwise.
  */
 element.isDOMElement = function(node) {
-  return "nodeType" in node && node.nodeType == node.ELEMENT_NODE;
+  return typeof node == "object" &&
+      "nodeType" in node &&
+      node.nodeType == node.ELEMENT_NODE;
 };
 
 /**
  * Ascertains whether <var>node</var> is a <code>WindowProxy</code>.
  *
  * @param {*} node
  *     Node thought to be a <code>WindowProxy</code>.
  *