SimpleConstraint problem using its int constructor

I'm migrating from zk 7.0.8.1 to 8.6.0.1 and I'm facing a problem with SimpleConstraint.
Unlike zk7 version, there is a problem instantiating a SimpleConstraint using its int constructor.
For example, I want a "no empty" constraint in a datebox.
If I create it in this way everything works fine --> constraintByString = new SimpleConstraint("no empty");
But if I create it in this way it doesn't work --> constraintByFlag = new SimpleConstraint(256);
See attachments for a simple example.