Discussion list owner email: 'Please correct the error below before continuing'

I’m trying to add a second list owner email to one of my discussion lists. The web panel doesn’t seem to like this: the only thing I can do is save the owner email list unchanged. Adding a second owner email or changing the existing email both give the following error:

Please correct the error below before continuing

There is no actual field error displayed in the form, so I can’t actually correct anything. The only other control on the page is ‘Delete “convenience email” addresses set up for this list’, which I don’t want to do!

My current owner email textbox looks like this:
webmaster@[my domain]

I’m trying to change it to:
webmaster@[my domain]
membership@[my domain]

Since this failed, I also tried changing it to:
membership@[my domain]

I’m not in a critical rush, but I’d love some help with this error, or even just confirmation that I’m not doing something really dumb :slight_smile: