WPMU schedule doesn't execute actions

WPMU schedule doesn't execute actions

Here's a simple event that is correctly scheduled to be executed daily.
Unfortuantely, it doesn't execute delete_old_users() function daily. It is
supposed to delete users older than a certain date.
function delete_old_users() {
$old_users = get_site_option( 'old_users' );
foreach ( $old_users as $user_id => $expiration_time ) {
if ( $expiration_time <= gmmktime() ) {
wpmu_delete_user( $user_id );
}
}
return;
}
add_action( 'delete_old_users_daily', 'delete_old_users' );
function setup_schedule() {
if ( ! wp_next_scheduled( 'delete_old_users_daily' ) ) {
wp_schedule_event( time(), 'daily', 'delete_old_users_daily');
}
}
add_action( 'wp', 'setup_schedule' );
Is there anything I'm missing here? When I manually do do_action(
'delete_old_users_daily' ) it works. I'm out of ideas. It just doesn't
execute my action and I have no idea how to debug that.