Oracle Forms

hi all,
i have a function get_conversion()
four in parameter and one out parameter. hwo to call that function inside the loop;
i called like
select get_conversion(company,from_uom,to_uom,value,err_msg) into variable1 from dual;
and writing the variable1 into the output file.
except 'err_msg' other parameters are in parameters.
but i am getting the ora-06572(i have written the code in the when-button-pressed trigger).
Please help me how to call a procedure.

Thanks..