empty role names in pg_dumpall output

Hi,

PostgreSQL 9.0.4

I have this in pg_dumpall -g output (non-empty role names changed):

GRANT "" TO a GRANTED BY postgres;
GRANT "" TO b GRANTED BY c;
GRANT "" TO b GRANTED BY c;
GRANT "" TO b GRANTED BY c;
GRANT "" TO b GRANTED BY c;
GRANT "" TO "" GRANTED BY c;
GRANT "" TO "" GRANTED BY postgres;
GRANT "" TO "" GRANTED BY postgres;

There is no CREATE ROLE "" ... in the output.

select * from pg_authid where length(rolname)<1 -- returns no rows

I would like to get rid of these roles, but how?

I appreciate any hints / ideas.