serialization a fileupload

i have a problem where my fileupload gave an error as below image. how do i fix it.. i attach my code as well below. please someone help me. My session state is InProc.

 

 

 

aspx.cs code :

 

if (Session["FileUploadPerjanjian"] == null && FileUploadPerjanjian.HasFile)
{
Session["FileUploadPerjanjian"] = FileUploadPerjanjian;
lblperjanjian.Text = FileUploadPerjanjian.FileName;
}
//// Next time submit and Session has values but FileUpload is Blank
//// Return the values from session to FileUpload
else if (Session["FileUploadPerjanjian"] != null && (!FileUploadPerjanjian.HasFile))
{
FileUploadPerjanjian = (FileUpload)Session["FileUploadPerjanjian"];
lblperjanjian.Text = FileUploadPerjanjian.FileName;
}
//// Now there could be another sictution when Session has File but user want to change the file
//// In this case we have to change the file in session object
else if (FileUploadPerjanjian.HasFile)
{
Session["FileUploadPerjanjian"] = FileUploadPerjanjian;
lblperjanjian.Text = FileUploadPerjanjian.FileName;
}