--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -745,34 +745,48 @@ element.generateUUID = function() {
 
 /**
  * This function generates a pair of coordinates relative to the viewport
  * given a target element and coordinates relative to that element's
  * top-left corner.
  *
  * @param {Node} node
  *     Target node.
- * @param {number=} x
- *     Horizontal offset relative to target.  Defaults to the centre of
- *     the target's bounding box.
- * @param {number=} y
- *     Vertical offset relative to target.  Defaults to the centre of
- *     the target's bounding box.
+ * @param {number=} xOffset
+ *     Horizontal offset relative to target's top-left corner.
+ *     Defaults to the centre of the target's bounding box.
+ * @param {number=} yOffset
+ *     Vertical offset relative to target's top-left corner.  Defaults to
+ *     the centre of the target's bounding box.
+ *
+ * @return {Object.<string, number>}
+ *     X- and Y coordinates.
+ *
+ * @throws TypeError
+ *     If |xOffset| or |yOffset| are not numbers.
  */
-element.coordinates = function(node, x = undefined, y = undefined) {
+element.coordinates = function(
+    node, xOffset = undefined, yOffset = undefined) {
+
   let box = node.getBoundingClientRect();
-  if (!x) {
-    x = box.width / 2.0;
+
+  if (typeof xOffset == "undefined" || xOffset === null) {
+    xOffset = box.width / 2.0;
+  }
+  if (typeof yOffset == "undefined" || yOffset === null) {
+    yOffset = box.height / 2.0;
   }
-  if (!y) {
-    y = box.height / 2.0;
+
+  if (typeof yOffset != "number" || typeof xOffset != "number") {
+    throw new TypeError("Offset must be a number");
   }
+
   return {
-    x: box.left + x,
-    y: box.top + y,
+    x: box.left + xOffset,
+    y: box.top + yOffset,
   };
 };
 
 /**
  * This function returns true if the node is in the viewport.
  *
  * @param {Element} element
  *     Target element.

new file mode 100644
--- /dev/null
+++ b/testing/marionette/test_element.js
@@ -0,0 +1,44 @@
+/* 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/. */
+
+const {utils: Cu} = Components;
+
+Cu.import("chrome://marionette/content/element.js");
+
+let el = {
+  getBoundingClientRect: function() {
+    return {
+      top: 0,
+      left: 0,
+      width: 100,
+      height: 100,
+    };
+  }
+};
+
+add_test(function test_coordinates() {
+  let p = element.coordinates(el);
+  ok(p.hasOwnProperty("x"));
+  ok(p.hasOwnProperty("y"));
+  equal("number", typeof p.x);
+  equal("number", typeof p.y);
+
+  deepEqual({x: 50, y: 50}, element.coordinates(el));
+  deepEqual({x: 10, y: 10}, element.coordinates(el, 10, 10));
+  deepEqual({x: -5, y: -5}, element.coordinates(el, -5, -5));
+
+  Assert.throws(() => element.coordinates(null));
+
+  Assert.throws(() => element.coordinates(el, "string", undefined));
+  Assert.throws(() => element.coordinates(el, undefined, "string"));
+  Assert.throws(() => element.coordinates(el, "string", "string"));
+  Assert.throws(() => element.coordinates(el, {}, undefined));
+  Assert.throws(() => element.coordinates(el, undefined, {}));
+  Assert.throws(() => element.coordinates(el, {}, {}));
+  Assert.throws(() => element.coordinates(el, [], undefined));
+  Assert.throws(() => element.coordinates(el, undefined, []));
+  Assert.throws(() => element.coordinates(el, [], []));
+
+  run_next_test();
+});

--- a/testing/marionette/unit.ini
+++ b/testing/marionette/unit.ini
@@ -2,10 +2,11 @@
 # 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/.
 
 # xpcshell unit tests for Marionette
 
 [DEFAULT]
 skip-if = appname == "thunderbird"
 
+[test_element.js]
 [test_error.js]
 [test_message.js]