warning: implode(): Bad arguments. in /public_html/modules/user.module

The Private Message module in Drupal 4.7 seems to be causing the user.module to generate a database error as shown below. After tweaking user.module to correct the cookie persistence issue in a related document, I started receiving the error below after installing the private message.module

warning: array_keys(): The first argument should be an array in /home/mydomain/public_html/modules/user.module on line 695.
warning: implode(): Bad arguments. in /home/mydomain/public_html/modules/user.module on line 695.
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 /home/mydomain/public_html/includes/database.mysql.inc on line 120.

The watchdog log also has a line that expands into this message:

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 /home/mydomain/public_html/includes/database.mysql.inc on line 120.

Initially, I tried to examine the user.module since it wa mentioned in the error message, but I did not udnersyand how it was related. I downloaded a fresh copy of the module and loaded it to the server, but that did not solve the problem.

Procedure for a solution

I decided to progressively disable the installed modules until I could see when the problem ent away. Sure enough, removing most of the added modules made the problem disappear. I added one by one until it re-appeared and that's how I decided that privatemsg.module was causing the problem.