Happens for both XML and JSON; in JSON, "track-count: null" is emitted, and in XML, there is no "track-count" attribute on the <track-list>.

The problem seems to be that both the CDStubTOC and CDStub have a track_count property; the one from CDStub is serialized, but it's the one from CDStubTOC that contains the actual track count.

Because I don't know whether or not the track_count from CDStub is ever really used, I'll just add a fallback to the other one.