Use of RETURN in pl/pgsql function

My script is below.

I thought (based on recent posts) that this use of RETURN is allowed, but
when trying an insert to report_table, I get the following error:

ERROR:  control reaches end of trigger procedure without RETURN

I have solved several problems in getting to this point, but have now run
out of ideas.  I would appreciate any pointers.

jeffe@kiyoko=> uname -a
FreeBSD kiyoko.la.verio.net 4.0-STABLE FreeBSD 4.0-STABLE #0: Thu Apr 27
10:44:07 CDT 2000
jeffe@kiyoko=> psql -V
psql (PostgreSQL) 7.0.0

Script:

drop function mrr();

create function mrr() returns opaque as '

begin

if NEW.billing_frequency = ''Monthly''          -- That's doubled single
quotes (and below as well)

then
       return NEW;

else
       if NEW.billing_frequency = ''Yearly''
       then
               NEW.rate := NEW.rate/12;               NEW.rate_override := NEW.rate_override/12;
               return NEW;
       else
               if NEW.billing_frequency = ''Semi-Annual''
               then
                       NEW.rate := NEW.rate/6;                       NEW.rate_override := NEW.rate_override/6;
                   return NEW;
 
               else 
                       if NEW.billing_frequency = ''Quarterly''
                       then
                               NEW.rate := NEW.rate/3;
                               NEW.rate_override := NEW.rate_override/3;
                               return NEW;
                       end if;
               end if;
       end if;

end if;

end;

'language 'plpgsql';

drop trigger mrr_set_trigger on report_table;

create trigger mrr_set_trigger 

before insert on report_table

for each row execute procedure mrr();