OK, I have found it now. Sorry for my hasty posts, I want to start my weekend...
The trouble starts when I annotate a private method with @AssertTrue:

@AssertTrue
private boolean getFoo()
{
    return true;
}

It was probably somewhat silly to use the annotation with a private method, but I can't find anything on it in the reference doc. And it certainly should not throw NullPointerExceptions.