greatest and coalesce on null date does not produce null

I create a table and insert one row like this:

create table datetest (nr decimal(1), date1 date);

insert into datetest values (1,null);

The bug can be reproduced with this statement:

least(coalesce(date1), coalesce(date1)) 

this should return null, but it doesnt.
E.g.

select * from datetest where least(coalesce(date1), coalesce(date1)) is not null; 

==> Returns one row

It is also possible to insert this "corrupt null" into a table:

insert into datetest select 2, least(coalesce(date1), coalesce(date1))  from datetest;

if you select from this table with a select * from datetest where date1 is not null, you will get the row with nr=2

Additional info:
I get the same error using "greatest" instead of "least".
I get the same error if I use more than one parameter for coalesce.
I do not get this error if I use another datataype, like varchar instead of date.
I do not get this error on another database like oracle 11g.