Exception due to the part

Hi,
I don't know why I get this exception
Data type mismatch in criteria expression.Microsoft Access Database Engine

once I've added this part

(@cat=0 or ent_id=@cat)

to the conditions below

                cmd = new OleDbCommand("SELECT ite_abbr & ' ' & ite_name as row1 FROM ite_master where (@cat=0 or ent_id=@cat) and (@int='False' or @int='True' and ite_id in(select ite_id from ite_tab where int_id>11 and int_id in(select int_id from ite_master a,ite_tab b where a.ite_id=b.ite_id and ite_abbr=@abbr))) and ite_id not in(select ite_id from ite_master where ite_abbr=@abbr) order by 1", conn);
                if (ent_cat_ddl.SelectedIndex > 0)
                    cmd.Parameters.Add("@cat", OleDbType.Decimal).Value = Convert.ToDecimal(ent_cat_ddl.SelectedValue);
                else
                    cmd.Parameters.Add("@cat", OleDbType.Decimal).Value = 0;
                ...

                reader = cmd.ExecuteReader();

ent_id is a Number column in the table. If I've removed the part, then it is fine.

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Many Thanks & Best Regards, Hua Min