apt-get update produces the error

[Grizzly]

Hi

When I run "apt-get update" it sems to work but ends with

W: The repository 'http://extras.ubuntu.com/ubuntu xenial Release' does not have a Release file.
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.
E: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/source/Sources  404  Not Found [IP: 91.189.92.152 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

IP 91.189.92.152 appears to to be correct, not sure about the 80 on the end or 
where it got that from (a missing ":" maybe but where?)

Did I miss something?