NAB : insert into <table> select distinct <fields> => when used on null, distinct causes loss of type knowledge

Hi,

Not exactly a showstopper, but I noticed this behaviour:

db=# create table f1 (id int, value int);
CREATE TABLE

db=# insert into f1 select 1 as id, null;
INSERT 25456306 1

db=# insert into f1 select distinct 2 as id, null;
ERROR: column "value" is of type integer but expression is of type text
HINT: You will need to rewrite or cast the expression.

db=# insert into f1 select distinct on (id) 2 as id, null;
INSERT 25456307 1

So it seems distinct applied to the second column causes it to lose knowledge
on its type.

Does anybody happen to know why ?

--
Best,

Frank.