WatiN Cannot find window class

Has anyone experienced the following error? It seems intermittent but can't track the problem down.

System.Runtime.InteropServices.COMException (0x8007057F): Cannot find window class. (Exception from HRESULT: 0x8007057F)     
at mshtml.HTMLDocumentClass.IHTMLDocument2_get_parentWindow()     at WatiN.Core.Native.InternetExplorer.IEElement.get_ParentWindow()     
at WatiN.Core.Native.InternetExplorer.IEElement.GetJavaScriptElementReference()     
at WatiN.Core.Native.InternetExplorer.IEFireEventHandler.CreateJavaScriptFireEventCode(NameValueCollection eventObjectProperties, String eventName)     
at WatiN.Core.Native.InternetExplorer.IEFireEventHandler.FireEvent(String eventName, NameValueCollection eventProperties, Boolean noWait)     
at WatiN.Core.Native.InternetExplorer.IEElement.FireEvent(String eventName, NameValueCollection eventProperties)     
at WatiN.Core.Element.FireEvent(String eventName, Boolean waitForComplete, NameValueCollection eventProperties)     

Edit: This is on Windows 7 using IE9