Array merge error on cron

When inactive user module is active, I get the message below. When I deactivate the module, I no longer get the message in the logs. To my untrained eye it looks like it could be a php4 vs php5 issue around how array merge works but that would require quite a few changes?

Invalid argument supplied for foreach() in /home/xxxxxxxx/public_html/includes/common.inc on line 1983.
array_merge() [function.array-merge]: Argument #2 is not an array in /home/xxxxxxxx/public_html/includes/common.inc on line 1952.

Drupal 5.7

Configuration file Protected
Cron maintenance tasks Last run 24 min 41 sec ago
You can run cron manually.
Database schema Up to date
File system Writable (public download method)
GD library bundled (2.0.34 compatible)
JQuery Update Installed correctly
The current installed version of JQuery is 1.1.2
MySQL database 5.0.51a
PHP 5.2.5
PHP register globals Disabled
Unicode library PHP Mbstring Extension
Web server Apache/1.3.37 (Unix) mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 FrontPage/5.0.2.2635.SR1.2 mod_ssl/2.8.28 OpenSSL/0.9.7a PHP-CGI/0.4mm