False error on the way to WAN CARP

[MrPete]

I'm setting up WAN CARP.
My supplies WAN as: Gigabit fiber with static IP over PPPoE (oh joy ;) ) ... I get a nice simple cat6 ethernet connector.

Time to set things up. I already have a virtual interface with a different VLAN. I would like to get this mostly configured without breaking what already works.

So, let's go create a VLAN to the WAN interface.

Immediate error: "A VLAN with the tag 203 is already defined on this interface."
Except:
a) That's not true. The existing VLAN is 201
b) I can try other tags and always get the same error
c) ifconfig shows that the new vlan interfaces were actually created! (vtnet0.203 etc)

Sure feels like a bug.