Can't perform SEO with 2 Zones

[Craig Bagley]

See screen shot below:

When I try to eliminate a small overlap between 2 Zones using one as the Operator and one as the Target, I get the warning message below.

The Zones are not locked, or on locked layers. It's a simple .pln file. I find no explanation of the warning anywhere.

Can anyone explain what's triggering this warning?

Craig