ERROR: plpgsql: cache lookup from pg_proc failed

If I run the following script:

create function functst() returns int4 as '
begin
return 1;
end;
' language 'plpgsql';
select functst() as rc;

I get:

DROP
CREATE
ERROR: plpgsql: cache lookup from pg_proc failed

Could someone explain what is causing this error?

TIA,
-km