Strange Warning: "Couldn't add re-parsed router: Some certs on this router are expired."

Today I updated my bridge to Tor 0.2.8.7 due to the upcoming bridge authority switch.

After some hours of operation I see in the log the following warning:

"Couldn't add re-parsed router: Some certs on this router are expired."

Can someone please explain what this warning means. Is this something I have to worry about?

Trac:
Username: torland