PostgreSQL 9.0.3 cannot choose the best candidate function with type casts

Good day,

I have a query in which I'm using a function called fx_get_parcel() like this:
SELECT fx_get_parcel(cast('cash' AS varchar), cast(rs_i.d_depot AS varchar)) FROM...
Without the type casts, Postgres complains that I have to add them. When I add them, the query fails and I get this
hint:
HINT: Could not choose a best candidate function. You might need to add explicit type casts.

The function has two versions:
fx_get_parcel(character varying, character varying)
and
fx_get_parcel(character varying, character varying, integer)

Both are in plpgsql.
Any ideas on how I could make it choose the first version with two parameters?
I've tried to find it in the documentation but I must have missed it somewhere.

I'm running PostgreSQL 9.0.3 on 32-bit Windows XP.

Thanks in advance. :)

Peter