Howdy colewebdev,
Thanks for taking the time to report this. That “error” is actually not much of an error at all, which is why at the top of the validator you will see a big green “This document was successfully checked as well-formed XML!”.
You can see this error in all WordPress feeds, including the calendar. It’s basically a part of WordPress. If you try validating the base WordPress feed you will see the error there too. In order to fix this error WordPress itself will need to be patched. I believe the WordPress devs have no intention of fixing it for a variety of reasons, one of which is that it’s harmless.
So you should be all set. Assuming Constant Contact can import events in the method your are trying, then you are good to go.
Does that all make sense? Does that address your issue? Please let me know.
Cheers!
– Brook