changing from bullseye to bookworm to install mmsd-tng

[jyoung]

I'm trying to change from the bullseye repository to the bookworm, and I'm getting an error about the repository being unsigned. In /etc/apt/sources.list.d/mobian.list

I changed

deb http://repo.mobian-project.org/ bullseye main non-free

to

deb http://repo.mobian-project.org/ bookworm main non-free

but now when I run

apt update

I get

Hit:1 http://deb.debian.org/debian bullseye InRelease
Get:2 http://repo.mobian-project.org bookworm InRelease [32.4 kB]
Err:2 http://repo.mobian-project.org bookworm InRelease
  The following signatures were invalid: EXPKEYSIG 951D61F2BC232697 Mobian Project <admin@mobian-project.org>
Reading package lists... Done
W: GPG error: http://repo.mobian-project.org bookworm InRelease: The following signatures were invalid: EXPKEYSIG 951D61F2BC232697 Mobian Project <admin@mobian-project.org>
E: The repository 'http://repo.mobian-project.org bookworm InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Does anyone know what might cause this?