--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1250,17 +1250,18 @@ GeckoDriver.prototype.getWindowPosition 
  *     Object with |x| and |y| coordinates.
  */
 GeckoDriver.prototype.setWindowPosition = function(cmd, resp) {
   if (this.appName != "Firefox") {
     throw new UnsupportedOperationError("Unable to set the window position on mobile");
   }
 
   let {x, y} = cmd.parameters;
-  if (!Number.isInteger(x) || !Number.isInteger(y)) {
+  if (!Number.isInteger(x) || !Number.isInteger(y) ||
+      x < 0 || y < 0) {
     throw new InvalidArgumentError();
   }
 
   let win = this.getCurrentWindow();
   win.moveTo(x, y);
 
   return this.curBrowser.position;
 };

--- a/testing/marionette/harness/marionette/tests/unit/test_window_position.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_window_position.py
@@ -21,14 +21,20 @@ class TestWindowPosition(MarionetteTestC
         self.assertTrue(isinstance(position["x"], int))
         self.assertTrue(isinstance(position["y"], int))
 
     def test_set_types(self):
         for x, y in (["a", "b"], [1.2, 3.4], [True, False], [[], []], [{}, {}]):
             with self.assertRaises(InvalidArgumentException):
                 self.marionette.set_window_position(x, y)
 
+    def test_out_of_bounds_arguments(self):
+        with self.assertRaises(InvalidArgumentException):
+            self.marionette.set_window_position(-1, 0)
+        with self.assertRaises(InvalidArgumentException):
+            self.marionette.set_window_position(0, -1)
+
     def test_move(self):
         old_position = self.marionette.get_window_position()
         new_position = {"x": old_position["x"] + 10, "y": old_position["y"] + 10}
         self.marionette.set_window_position(new_position["x"], new_position["y"])
         self.assertNotEqual(old_position['x'], new_position["x"])
         self.assertNotEqual(old_position['y'], new_position["y"])