Create UDA does not recognize built-in functions without specifying system keyspace

I'd expect when creating a UDA not to have to specify the system keyspace when using a built-in function for the state or final function. For example:

CREATE AGGREGATE if not exists ks.my_agg(int) 
SFUNC ks.adder
STYPE int
FINALFUNC intasblob
;

Gives me me the error:

Final function ks.intasblob(int) does not exist or is not a scalar function

Prefixing with 'system.' fixes the issue. But I was expecting it to resolve as if I were using a built in function elsewhere.