Unique Field triggering on "node edit," is this correct?

I've got a content type in which I've set up pairs of fields must be unique across all nodes of this content type: basically a "date" and an "event." So like..."Spring 2014 Party" would be the pair "Spring 2014" and "Party", right?

When we initially create the node, everything works great. You can't create the same pair. However, when you go in and edit the node, it keeps you from saving the node. We're absolutely not creating a NEW node, but Unique Field thinks we're trying to create a new node and the save fails.

We can hit "bypass" and resave, but that's a hassle and ugly to the end user.

Is this the expected behavior? Is there anything we can do to make Unique Field only evaluate new nodes, not existing nodes?

Thanks!