Specified cast is not valid. error in using linq-to- sql class in c#

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C#

LINQ

I am using linq-to-sql to connect my web application to sql server... my code is.

EmployeeCheckoutDataContext db = new EmployeeCheckoutDataContext();
EmployeeDetail ed = null;

protected void Button1_Click(object sender, EventArgs e)
{
    ed = (from a in db.GetTable<EmployeeDetail>()
          where (a.Name == "xyz")
                select a).SingleOrDefault<EmployeeDetail>();
    Label1.Text = ed.EmployeeNo;
}

as i compile the code, the error i get is "Specified cast is not valid." at code

ed = (from a in db.GetTable<EmployeeDetail>()
         where (a.Name == "xyz")
               select a).SingleOrDefault<EmployeeDetail>();

i had also tried with var type. i am completely blank to how this is happening.. any help??