Error trigger

I have a trigger when I executed I get this error:

WARNING:  Error occurred while executing PL/pgSQL function
actualizar_pagos_comisiones
WARNING:  line 19 at select into variables
ERROR:  Unable to identify an operator '=' for types 'character' and
'character varying'
        You will have to retype this query using an explicit cast

who varchar;

who:=cast($1 as varchar) || $2;

The line 19 is this
SELECT INTO registro * from t_pagos_comisiones where f_wholetipoagnivel =
who and f_fecha = $4;


*-------------------------------------------------------*
*-Edwin Quijada
*-Developer DataBase
*-JQ Microsistemas
*-809-747-2787
* " Si deseas lograr cosas excepcionales debes de hacer cosas fuera de lo
comun"
*-------------------------------------------------------*

_________________________________________________________________
¿Estás buscando un auto nuevo?  http://www.yupimsn.com/autos/