Custom Module's Related Contacts not Showing in Email Compose Address Book

I’ve built two custom modules with the module builder, and am having the same problem with both of them.

When going to Emails -> Compose, and selecting the custom module and a record as “related to”, then clicking on the “TO:” button, I get a “Data Error.” message in the address book where the contact list should be. The OOB modules, however, all work correctly.

How do I go about making the related contacts for my custom modules available in the emails address book?

I noticed in the Chrome developer tools’ Network tab when trying to load the Select Email Recipients window for the custom module I get a 500 error when it tries to call the following:

http://***/index.php?to_pdf=true&module=Emails&action=EmailUIAjax&emailUIAction=getAddressSearchResults&related_bean_id=501b092a-28f3-337f-a11a-5485074dc72c&related_bean_type=vend_Vendors&person=LBL_DROPDOWN_LIST_ALL&search_field=&

For comparison, here is a working one:

http://***/index.php?to_pdf=true&module=Emails&action=EmailUIAjax&emailUIAction=getAddressSearchResults&related_bean_id=9b5c8363-2b78-5519-b31b-5465276e80f9&related_bean_type=Accounts&person=LBL_DROPDOWN_LIST_ALL&search_field=&