Windows Phone Null URI Exception for WebView

The WebView control throws an exception saying "URI is null or empty" on Windows, but it doesn't seem to be a problem for iOS or Android. I want the URL to be empty so that I can load HTML that is persisted in a local database. If I set the SourceProperty the exception never gets thrown, but now I can't get the page to refresh to display the HTML. Has anyone had a similar issue, or knows how to refresh the WebView control to display the local HTML?

...
            htmlSource.SetBinding (HtmlWebViewSource.HtmlProperty, "Html");
...            
            if (Device.OS == TargetPlatform.WinPhone)
            {
                webView.SetValue (WebView.SourceProperty, "about:blank");
            }

Thanks