ASP.NET session has expired or could not be found

I have a Report Viewer connected to an SSRS report on a SQL Server.  I setup error logging in the global.asax so it sends me an email.  I keep getting these errors with my report viewer.  The user is unaffected and never sees the error occur.  Its like the IIS sessions having some issue?  Anyone ever seen this?

Error Caught in Application_Error event
Error in: http://sales.test.com/Reserved.ReportViewerWebControl.axd?OpType=SessionKeepAlive&ControlID=42d3fa58b1b2401983ee1e5ebc4da677
Error Message:ASP.NET session has expired or could not be found
Stack Trace:   at Microsoft.Reporting.WebForms.ViewerDataOperation..ctor()
   at Microsoft.Reporting.WebForms.HttpHandler.GetHandler(String operationType)
   at Microsoft.Reporting.WebForms.HttpHandler.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)