How to update an access table(one column have autonumber) in C#?

Hello, I have an MS ACCESS table with two columns named item_nm(text) and item_id(This column is autonumber). Whenever I"m trying to update table it's getting an error("Data type mismatch in criteria expression"). Please solve this quest. Thanks!!!!

private void button2_Click(object sender, EventArgs e)
        {
                cn.Open();
                OleDbCommand cm = new OleDbCommand("update tbl_purchaseitementry set item_nm='@item_nm' where item_id='@item_id'", cn);
                cm.Parameters.AddWithValue("@item_nm",txt_itemname.Text);
                cm.Parameters.AddWithValue("@item_id",txt_itemid.Text);
                OleDbDataAdapter da = new OleDbDataAdapter(cm);
                cm.ExecuteNonQuery();
                cn.Close();
        }

Tags: C# (C# 4.0)