[graph-tool] Public key not available

Hello, When updating via apt-get on Ubuntu 16.04 I get an error message saying: Err:20 http://downloads.skewed.de/apt/xenial 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. Best, Philipp -- Sent from: http://main-discussion-list-for-the-graph-tool-project.982480.n3.nabble.com/