When a student tries to use quickmail they get the following error:

"Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays" is displayed.

Turning debugging on displays this additional information:

Stack trace:

line 588 of /lib/dml/moodle_database.php: coding_exception thrown
line 86 of /blocks/quickmail/email.php: call to moodle_database->get_in_or_equal()

We have replicated this error on several sites. Also a teacher in a course that they are not a teacher in, also gets this error when attempting to use quickmail.

Steps to reproduce:
Go to a course within a site, login as the test student. In Quickmail block, click "Compose New Email". The error "Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays" is displayed.