Coded ui ExecuteScript method is returning null when i run document.readyState from my browser object

I'm executing the below code but its returning null. I don't know why.
When i perform the document.readyState on the browser it works. If someone can point me in the right direction, it would be greatly appreciated.

 object documentState;
        // check to see if browser window is finished loading
        do
        {
            Playback.Wait(1000);
            documentState = _CurrentBrowser.ExecuteScript("var myState = document.readyState;");

        } while (documentState.ToString() !="complete");