diff --git a/lib/FrameManager.js b/lib/FrameManager.js
index 5c854102..cfca89a8 100644
--- a/lib/FrameManager.js
+++ b/lib/FrameManager.js
@@ -367,7 +367,9 @@ class Frame {
     this._loaderId = '';
     /** @type {!Set<string>} */
     this._lifecycleEvents = new Set();
+    /** @type {!DOMWorld} */
     this._mainWorld = new DOMWorld(frameManager, this, frameManager._timeoutSettings);
+    /** @type {!DOMWorld} */
     this._secondaryWorld = new DOMWorld(frameManager, this, frameManager._timeoutSettings);
 
     /** @type {!Set<!Frame>} */