Search Records "in" Criteria Responds Empty

Search Records "in" Criteria Responds Empty

I am looking up multiple records in a linking module, linking Contacts and Accounts (which I've renamed to Entities) called Entity_Members. I want to get a list of matching Account IDs based on a list of Contact IDs through that linking module. I was thinking I could use searchRecords and set the criteria = "Entity_Members:in:" + contact_id_list. Entity_Members is the linking module field name for the Contacts lookup. However that criteria responds with nothing, whereas if I loop over the contact list and call searchRecords each time with criteria = "Entity_Members:equals:" + contact_id, it works. 

Any reason why the "in" operator wouldn't be working?