how to return the JSON of a javascript object in a WPF application

how to return the JSON of a javascript object in a WPF application

I have a WPF app with a WebBrowser control
I need to get the JSON string of a javascript object, something like this:
var json = this.WebBrowser.InvokeScript("eval", new object[] {
"JSON.stringify(window.mainThing)" });
I am executing this code on WebBrowser's LoadCompleted event
this throws an exception
System.Runtime.InteropServices.COMException was unhandled by user code
HResult=-2147352319
Message=Exception from HRESULT: 0x80020101
Source=WindowsBase
ErrorCode=-2147352319
StackTrace:
at MS.Internal.Interop.HRESULT.ThrowIfFailed(String message)
at System.Windows.Controls.WebBrowser.InvokeScript(String
scriptName, Object[] args)
Here's the Javascript in the page:
<script type="text/javascript">
window.mainThing = { x : 1 };
</script>