VcVirtualMachineSnapshotTree.snapshot is undefined for a given VcVirtualMachine

I'm running the vCO 5.1.1 appliance, and using the stock action to return snapshots (getAllSnapshotsOfVm).  Running the action against any given VM with at least one snapshot produces an array of undefined object(s).

It's as if the snapshot property doesn't exist on the VcVirtualMachineSnapshotTree object for which the action getAllSnapshotsOfVm recursively searches from <vm>.snapshot.rootSnapshotList.

In the log it shows:

2013-10-22 16:14:10.238+0000 WARN  [DynamicWrapper] Property named 'snapshot' not found on object : VcVirtualMachineSnapshotTree

Any ideas?

Thanks,

David