diff --git a/lib/frame.js b/lib/frame.js
index a9b31a9..c5c9d00 100644
--- a/lib/frame.js
+++ b/lib/frame.js
@@ -81,10 +81,6 @@
 	
 	var interceptedWindows = new WeakMap();
 	function interceptWindow(window){
-		if (!enabled || interceptedWindows.get(window)){
-			return false;
-		}
-		
 		try {
 			var href = window.location.href;
 		}
@@ -95,6 +91,10 @@
 			return false;
 		}
 		
+		if (!enabled || interceptedWindows.get(getWrapped(window))){
+			return false;
+		}
+		
 		message("intercepting window", window);
 		intercept(
 			{subject: window},
@@ -112,7 +112,9 @@
 					configureable: true,
 					get: exportFunction(function(){
 						var window = oldContentWindowGetter.call(this);
-						interceptWindow(window);
+						if (window){
+							interceptWindow(window);
+						}
 						return window;
 					}, window)
 				}
@@ -126,14 +128,16 @@
 					configureable: true,
 					get: exportFunction(function(){
 						var document = oldContentDocumentGetter.call(this);
-						interceptWindow(document.defaultView);
+						if (document){
+							interceptWindow(document.defaultView);
+						}
 						return document;
 					}, window)
 				}
 			);
 		});
 		
-		interceptedWindows.set(window, true);
+		interceptedWindows.set(getWrapped(window), true);
 		return true;
 	}