--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1309,16 +1309,17 @@ GeckoDriver.prototype.getChromeWindowHan
  *     of browser window.
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getWindowRect = function(cmd, resp) {
+  assert.window(this.getCurrentWindow());
   assert.noUserPrompt(this.dialog);
   return this.curBrowser.rect;
 };
 
 /**
  * Set the window position and size of the browser on the operating
  * system window manager.
  *