Dropping users with no name

I have accidentally created a user with no name. How can I delete this
user?
I have compiled my attempts below:
   Hans


persons=# SELECT * FROM pg_user;usename  | usesysid | usecreatedb | usetrace | usesuper | usecatupd |
passwd  |
valuntil
----------+----------+-------------+----------+----------+-----------+----------+----------
postgres |       26 | t           | t        | t        | t         |
******** |         |       27 | f           | f        | f        | f         |
******** |
(2 rows)

persons=# DELETE FROM pg_user where usesysid>26;
DELETE 0

persons=# SELECT '$'|| usename||'$' FROM pg_user; ?column?
------------$postgres$$$
(2 rows)

persons=# DROP USER '';
ERROR:  parser: parse error at or near "'"