Data Controls :: Updating Value From GridView Error

I get Object reference not set to an instance of an object                

cmd.CommandText ="UPDATE TB1 SET c_amt=@c_amt,c_type=@c_type,regby=@regby,dt=@dt WHERE  id=@ id";       cmd.Parameters.Add("@c_amt", SqlDbType.VarChar).Value = Convert.ToInt32(((TextBox)
Gridview1.Rows[e.RowIndex].Cells[2].Controls[0]).Text); 
cmd.Parameters.Add("@c_type", SqlDbType.VarChar).Value = ((TextBox)Gridview1.Rows[e.RowIndex].Cells[3].Controls[0]).Text;
cmd.Parameters.Add("@regby", SqlDbType.VarChar).Value = Session["name"]; 
cmd.Parameters.Add("@dt", SqlDbType.DateTime).Value = DateTime.Now.ToString("yyyy/MM/dd");UPDATE TBL1 SET c_amt='3000',c_type='E',regby='XYZ', dt='2012/05/24' WHERE id=94 

by using in query analyzer  i get it update but i had to put single quotes manually in date & amt ....