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;

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