Unknown Error

protected void Button4_Click(object sender, EventArgs e)
{
string selectedItem = DropDownList1.SelectedItem.Text;
if (selectedItem == "zezo")
{


string connectionString2 = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
SqlConnection connection2 = new SqlConnection(connectionString2);
connection2.Open();
SqlCommand command2 = new SqlCommand("finde", connection2);
command2.CommandType = CommandType.StoredProcedure;
command2.Parameters.Add("@columen_name", SqlDbType.NVarChar).Value = "taher";
command2.ExecuteNonQuery();
}

and this is the stored procedure

ALTER PROCEDURE dbo.finde @columen_name nvarchar(50)
/*
(
@parameter1 int = 5,
@parameter2 datatype OUTPUT
)
*/
AS
declare
@maxcount int,@sql varchar(8000)
set @maxcount=(select max(taher)+1 from counter)

set @sql='insert into counter (' + @columen_name +')
values ( @maxcount )'
exec (@sql)

************************
and this is the error

Server Error in '/NTSOLUTIONS' Application.
--------------------------------------------------------------------------------

Must declare the scalar variable "@maxcount".
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Must declare the scalar variable "@maxcount".

Source Error:


*********************************************
so i can not find where is the exact point

zezo