Xpra: Ticket #1828: GPG Key Expired

I received this message today from apt update:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://xpra.org xenial Release: The following signatures were invalid: KEYEXPIRED 1525416977  KEYEXPIRED 1525416977  KEYEXPIRED 1525416977
W: Failed to fetch https://xpra.org/dists/xenial/Release.gpg  The following signatures were invalid: KEYEXPIRED 1525416977  KEYEXPIRED 1525416977  KEYEXPIRED 1525416977
W: Some index files failed to download. They have been ignored, or old ones used instead.

I tried:

sudo apt-key -d 1525416977
curl https://xpra.org/gpg.asc | sudo apt-key add -

...but still get the same message.