Can't access sequence tracks or markers through ExtendScript

I'm using the following code in the ExtendScript Toolkit:

var seq = app.project.activeSequence;

alert(seq.name); // This returns the correct sequence name, so seq is not null

var tracks = seq.videoTracks;

alert(tracks.length); // This returns 'undefined' and is the start of my headaches

To confirm there are definitely video tracks on my sequence. I've tried this with new projects, new sequences, adding and deleting video tracks, locking and unlocking them but nothing seems to work. Have I missed something obvious?

Also, on a slightly related note, I am having problems accessing the markers on the sequence. The following seems bizarre:

var seq = app.project.activeSequence;

var markers = seq.markers;

alert(markers.length); // Returns 'undefined'

var newMarker = markers.createMarker(1); // Successfully adds a marker at one second.

alert(markers.length); // Returns 'undefined'

Thanks,

Pete