Data from such a repository can't be authenticated #25

I'm getting the following authentication error when trying to run your custom install steps for a Raspberry Pi.

W: GPG error: http://packages.namniart.com/repos/pi wheezy InRelease: The following signatures were invalid: 15A0B3807025FB9CC17EA6FF17BF1CB1D6FC98DF
W: The repository 'http://packages.namniart.com/repos/pi wheezy InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I did try to use the "--allow-unauthenticated" flag, but that didn't work either. I'm a Raspberry Pi and Python newbie, so I'm probably missing something easy.