stringtype=unspecified is null check problem

If a bind parameter is checked for null, ERROR: could not determine data type of parameter $1 occurs.
Simple testcase:
Connection conn = DriverManager.getConnection("jdbc:postgresql://192.168.1.201:5432/testdb?stringtype=unspecified",
"test","test"); 
PreparedStatement stat = conn.prepareStatement("select 1 where ? is null");
stat.setNull(1, Types.VARCHAR);
stat.executeQuery();
-> Exception

For checking the parameter for null, the data type should not be relevant. At least TEXT or VARCHAR could be implicitly
used.

There is a mailing post, that says, that stat.setNull(1, Types.VARCHAR); would strict set the value as VARCHAR.
https://www.postgresql.org/message-id/Pine.BSO.4.64.0702141439480.24142%40leary.csoft.net
But this seams not to happen in my case.

I have also testet to bind the value with:
stat.setNull(1, Types.NULL); or stat.setNull(1, Types.OTHER); or stat.setString(1, null);
Nothing works in this case.

As far as I understand, the jdbc driver sends the value without type information to the database. The database throws
theexception. 
So maybe this is a database problem.
The database can assume, that in case of ? is null it doesn't matter, which type the sent null is. So even a UNKOWN
nullis null or not null. 

Strange is, that the following statement will work. So in this case null is unknown, but mapped.
conn.prepareStatement("select 1 where coalesce(?, null) is null");