Error when trying to change netmask

[harlan]

I'm running 13.0-U51.

I've got a VLAN interface with the wrong netmask, 24, when it should be 16.

I go to: Network/Interfaces/Edit for the target VLAN, change the netmask to what it should be, and when I click on "Apply" I get an error popup:

! RuntimeError
2 found, expecting one

Ordinarily, I'd just try deleting the interface and re-adding it, except that it mostly works now and I don't want to risk making things worse...

How to debug/fix?

 

[harlan]

I fixed it by deleting the interface and re-adding it.