Int field empty in Autoname V12

I am using this format as autoname in a custom doctype:

format:{month}-{year}

When year is an int, it comes out empty like January-
Changing year format to data it works well January-2019

I am testing V12

ERPNext: v12.x.x-develop () (develop)

Frappe Framework: v12.x.x-develop () (develop)