Null pointer exception on .iterator() call

I'm getting a strange NullPointerException, evidently thrown by the following line of code:

Iterator<Note> it = notes.iterator();

I've checked, and at the time the java.util.TreeSet notes is always non-null (with 15 elements). The TreeSet API says nothing about iterator() throwing NullPointerExceptions. What else could be going here?

If notes is not null then:

• You are mistaken (never discount this option).
• The implementation of notes is broken.
• You are using the implementation incorrectly - for instance having multiple threads access it at once.