Using rowtype parameter

I'm trying to write a stored proc (in pl/Pgl) that can accept rowtypes as arguments:

CREATE or replace FUNCTION www_get_data(user_id "varchar", objectname "varchar", operation "varchar", primarykeyvalue anyelement, rowvalue anyelement)
RETURNS SETOF varchar[] AS
...

Whenever I try to typecast a rowtype in function call like this:

select www_get_data ('test','USERS','QUERY',CAST(('%','%','','','') as mytable),CAST(('%','%','','','') as mytable))

I get error message:

ERROR: could not determine actual argument type for polymorphic function "www_get_data"

What gives? I thought typecast should suffice? Are there any limitations on using composite data types for anyelement parms?

Peter