BUG #11266: failed to find conversion function from unknown to bigint

The following bug has been logged on the website:

Bug reference: 11266
Logged by: zoulx1982
Email address: zoulx1982(at)163(dot)com
PostgreSQL version: 9.4beta2
Operating system: Win7
Description:

when i migrate my application from oracle to postgrs,
i encounter a query raise error "failed to find conversion function from
unknown to bigint"
the original query is very complicated, so i summarize a simple testcase:

create table t1(a varchar(10));
create table t2(x varchar(10), y bigint);
select s.*
from
(select a, null b from t1
group by a
) s
union all
select x,y from t2;--ERROR

postgres=# select version();
version
----------------------------------------------------------------
PostgreSQL 9.5devel, compiled by Visual C++ build 1600, 32-bit
(1 row)

and when i remove the "group" clause, the query works well
or if i don't use sub-query, it can also work well, i.e.
select a, null b from t1 group by a union all select x,y from t2;
and
select s.* from (select a, null b from t1) s union all select x,y from t2;
is all OK.

is there parameters to avoid the issue without changing original sql,
or doe's somebody can fix the issue later ?

thanks.