ResIterator contains null URIs

[Uri Shani]

Issuing this statement: ResIterator iter = model.listSubjects(); for a 
Jena Model, 
and than iterating as in: 
                while (iter.hasNext()) {
                        String uri = iter.next().getURI();
                        System.out.Println(uri);
                }
produces a lot of nulls.
the model is an OntModel, accessed as a plain default Model.

Any explanation for that?

Thanks,
- Uri