Bulk delete fails with "invalid user" error

Moodle 2.7 from most recent git pull.

About 3500 users. I'm trying to clean up users who haven't accessed in a year. I set a filter and then select all and select "delete" bulk action. I get the confirmation page & then this error.

If I use fewer users, I can delete them - it appears to be when it involves multiple (but as few as 20-30 at a time) where some characteristic is met.

The moodle log says:
Tue Jun 03 10:12:39.103157 2014] [:error] [pid 28566] [client 67.176.78.209:59045] Default exception handler: Invalid user Debug: SELECT id FROM

WHERE id = ? AND deleted = ?\n[array (\n 0 => '666',\n 1 => 0,\n)]\nError code: invaliduser\n* line 1451 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown\n* line 1427 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()\n* line 6404 of /lib/accesslib.php: call to moodle_database->get_record()\n* line 2396 of /message/lib.php: call to context_user::instance()\n* line 2342 of /message/lib.php: call to message_mark_message_read()\n* line 4212 of /lib/moodlelib.php: call to message_move_userfrom_unread2read()\n* line 30 of /admin/user/user_bulk_delete.php: call to delete_user()\n, referer: https://moodle.cs.colorado.edu/admin/user/user_bulk_delete.php
root@moodle:/var/log/apache2#