Unhandled Exceptions thrown

[Winista]

I upgraded our web site from 1.1 to 2.0 and now I have started seeing
exception messages when a search engine crawler tries to crawl the page. It
does not matter what page it tries to access, the same exception is getting
thrown. I just can't figure out what the possible cause could be because
when a normal user accesses the pages there are no errors at all. Any hint
will be greatly appreciated.

Thanks

Error Message:Exception of type 'System.Web.HttpUnhandledException' was
thrown.

Stack Trace: at System.Web.UI.Page.HandleError(Exception e)

at System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint,
Boolean includeStagesAfterAsyncPoint)

at System.Web.UI.Page.ProcessRequest()

at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)

at System.Web.UI.Page.ProcessRequest(HttpContext context)

at ASP.mysite_prod101_aspx.ProcessRequest(HttpContext context) in
c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET
Files\root\c601d9e5\73d89876\App_Web_a9inei-d.17.cs:line 0

at
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()

at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously) Value cannot be null.

Parameter name: String