Public key not available


When updating via apt-get on Ubuntu 16.04 I get an error message saying:

Err:20 xenial Release.gpg
  The following signatures couldn't be verified because the public key is
not available: NO_PUBKEY 78AF38AD52B46290

Re-adding the keys as outlined in the installation instructions seems to fix
the problem but I wanted to check if this was expected behaviour.

Thanks for any advice in advance.



Yes, the key changed some time ago.