Cannot move existing content into a newly created group

I created a new group and want to move previously public content into the group. I am an administrative member of the group. When I edit the content, change its Groups Audience, and save, I get "Attempt to post to an illegal group."

First of all, this error message is needlessly alarming. Now that American citizens can be imprisoned indefinitely without trial, we need to find a different wording to use here, or people will think Drupal is reporting them to the FBI. I'm not joking.

But more to the point, the error is not correct. I can create new content in the group without problem. Why can't I move existing content into the group?

Thanks in advance for any light you can shed on this.