Compilation Error happen in converting lable text

hi

I want use thousand seprator for label so I used below code:

   if (Session["Total"] != null)
        {
            lblKol.Text = Session["Total"].ToString("N0");
        }

but this error happen:

Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. Compiler Error Message: CS1501: No overload for method 'ToString' takes 1 arguments Source Error:

Line 20:         if (Session["Total"] != null)
Line 21:         {
Line 22:             lblKol.Text = Session["Total"].ToString("N0");
Line 23: 
Line 24:         }

can you help me please...

Best regards

neda