--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1245,17 +1245,17 @@ GeckoDriver.prototype.getWindowPosition 
  *     X coordinate of the top/left of the window that it will be
  *     moved to.
  * @param {number} y
  *     Y coordinate of the top/left of the window that it will be
  *     moved to.
  */
 GeckoDriver.prototype.setWindowPosition = function(cmd, resp) {
   if (this.appName != "Firefox") {
-    throw new WebDriverError("Unable to set the window position on mobile");
+    throw new UnsupportedOperationError("Unable to set the window position on mobile");
   }
 
   let x = parseInt(cmd.parameters.x);
   let y  = parseInt(cmd.parameters.y);
   if (isNaN(x) || isNaN(y)) {
     throw new UnknownError("x and y arguments should be integers");
   }