apt - Update Manager Error - Authentication Error

When I run Update Manager I get this error:

This is what the error box says:

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://linux.dropbox.com lucid Release: The following signatures were invalid: NODATA 1 NODATA 2

W: Failed to fetch http://linux.dropbox.com/ubuntu/dists/lucid/Release

W: Some index files failed to download, they have been ignored, or old ones used instead.

• Click System>>Administration>>Software Sources


• Click the Download From menu and choose Main Server


• Type the following in terminal.
  sudo apt-get update


• Run the update manager again.