ArgumentNullException - parameter name: user

 I have a image resize module in my application which is not working. I am getting strange error messages:
System.ArgumentNullException was unhandled by user code
  Message="Value cannot be null.\r\nParameter name: user"
  Source="System.Web"
  ParamName="user"
  StackTrace:
       at System.Web.Security.UrlAuthorizationModule.CheckUrlAccessForPrincipal(String virtualPath, IPrincipal user, String verb)
       at fbs.ImageResizer.InterceptModule.CheckRequest_PostAuthorizeRequest(Object sender, EventArgs e) in D:\_SYL_Project\Projects\WebApplication\HttpModule\ImageResizer\InterceptModule.cs:line 157
       at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
       at System.Web.HttpApplication.ExecuteStep(IExecut ...