JSON constructors and window functions

I got a crash running the below query on the regression database:

"""
select pg_catalog.json_object_agg_unique(10,
            cast(ref_0.level2_no as int4)) 
            over (partition by ref_0.parent_no 
            order by ref_0.level2_no)
from public.transition_table_level2 as ref_0;
"""

Attached the backtrace.

PS: I'm cc'ing Andrew and Nikita because my feeling is that this is 
f4fb45d15c59d7add2e1b81a9d477d0119a9691a responsability. 

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL