--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1248,20 +1248,19 @@ GeckoDriver.prototype.getWindowPosition 
  *     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 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");
+  let {x, y} = cmd.parameters;
+  if (!Number.isInteger(x) || !Number.isInteger(y)) {
+    throw new InvalidArgumentError();
   }
 
   let win = this.getCurrentWindow();
   win.moveTo(x, y);
 };
 
 /**
  * Switch current top-level browsing context by name or server-assigned ID.