diff --git a/lib/NavigatorWatcher.js b/lib/NavigatorWatcher.js
index 4ea721e495e0e..4a1c9411e9cd7 100644
--- a/lib/NavigatorWatcher.js
+++ b/lib/NavigatorWatcher.js
@@ -60,7 +60,7 @@ class NavigatorWatcher {
   }
 
   /**
-   * @return {?Promise<?Error>}
+   * @return {!Promise<?Error>}
    */
   _createTimeoutPromise() {
     if (!this._timeout)