config.defaultPortConfig.vlan.vlanId returns 0

[Inputs]

DVSwitch ofType VC:VmwareDistributedVirtualSwitch

The code below always returns 0 for vlanId. 

Any thoughts on why?
Is there a different way to retrieve the vlanId for a DVPortGroup?

for(i in DVSwitch.portgroup)
{      System.debug(DVSwitch.portgroup[i].name);      if(DVSwitch.portgroup[i].config.defaultPortConfig.vlan instanceof VcVmwareDistributedVirtualSwitchVlanIdSpec){            var vlanId = DVSwitch.portgroup[i].config.defaultPortConfig.vlan.vlanId;            System.debug(vlanId);     }
}