vDS requirements in vSphere6

Set up a test host on ESXi 5.5-latest, vcsa appliance.  Upgrade VC and ESXi from 5.5 to 6.

The vDS is 5.5.0, and shows an 'upgrade available'.  Tell the vDS to upgrade, it marks the now-6 ESXi hosts as incompatible, and we can't proceed with 5.5->6 on the vDS.

I know that's not a showstopper, but it's WEIRD.  What requirements have changed with vDS that would prevent this?  We're trying to figure out if it's a testing anomaly or a real problem.

Thanks!