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 "'"