Did something change in version 9 with being able to search on custom fields on models (i.e. account.payment.term) ?

I created a new field "x_key" on account.payment.term and I can see the field exists in the database

but the XMLRPC returns invalid leaf on that field when i change my search to use the field 'name' it works fine so the problem seems to be with using my custom field. 

Do I need to do something special to expose the field to the XMLRPC in version 9? The same code works just fine in version 8.