Subject auto generated ID, default values???

I don't get it anymore, I'm trying Firebird 1 beta 2 (on Windows),
however I've some problems...

I created a field with an autoincrement ID and not Null, primary key
When I try to insert a record with no value in that specific field I
get an error that the field must contain a value... this is strange,
since I thought that the value would have been generated before
insert (as how the trigger is made).

When I give a value, and then look at the created record, I see that
the field value is one of the generator, which is ok.

Now, I made the field again and gave it a default value of 0. Now
when I try to create a new record, I get again the same error...
while now it should work, since I give a default value!

What am I doing wrong??