[support] Error in user.module

[Paul W]

Using 4.7.3, I get this error even when just going to the 
site without any login involved:

* warning: array_keys() [function.array-keys]: The first 
argument should be an array in 
/var/www/html/modules/user.module on line 351.
* warning: implode() [function.implode]: Bad arguments. in 
/var/www/html/modules/user.module on line 351.
* user warning: You have an error in your SQL syntax; check 
the manual that corresponds to your MySQL server version for 
the right syntax to use near ')' at line 1 query: SELECT 
DISTINCT(p.perm) FROM role r INNER JOIN permission p ON 
p.rid = r.rid WHERE r.rid IN () in 
/var/www/html/includes/database.mysql.inc on line 120.

Everything worked yesterday and I have changed nothing! This 
is very strange. Can anyone help?

Paul W