--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -266,17 +266,18 @@ browser.Context = class {
    * @param {string} uri
    *      URI to open.
    */
   addTab(uri) {
     return this.tabBrowser.addTab(uri, true);
   }
 
   /**
-   * Set the current tab and update remoteness tracking if a tabbrowser is available.
+   * Set the current tab and update remoteness tracking if a tabbrowser
+   * is available.
    *
    * @param {number=} index
    *     Tab index to switch to. If the parameter is undefined,
    *     the currently selected tab will be used.
    * @param {nsIDOMWindow=} win
    *     Switch to this window before selecting the tab.
    * @param {boolean=} focus
    *      A boolean value which determins whether to focus
@@ -444,19 +445,22 @@ browser.Windows = class extends Map {
 
   /**
    * Get the window object stored by provided |id|.
    *
    * @param {string} id
    *     Outer window ID.
    *
    * @return {Window}
-   *     Saved window object, or |undefined| if no window is stored by
-   *     provided |id|.
+   *     Saved window object.
+   *
+   * @throws {RangeError}
+   *     If |id| is not in the store.
    */
   get(id) {
     let wref = super.get(id);
-    if (wref) {
-      return wref.get();
+    if (!wref) {
+      throw new RangeError();
     }
+    return wref.get();
   }
 
 };