Can't update my system due to GPG error Clearsigned file isn't valid, got 'NODATA' [duplicate]

When I was running sudo apt-get update I got the error:

Ign http://br.archive.ubuntu.com trusty InRelease                              
E: GPG error: http://br.archive.ubuntu.com trusty InRelease: Clearsigned file isn't valid, got 'NODATA' (does the network require authentication?)

To solve that, first I tried:

wget https://dl.dropbox.com/u/8850924/fixpackage
chmod +x ./fixpackage
sudo ./fixpackage

As recomended in https://answers.launchpad.net/ubuntu/+question/251767

But it didn't work for me. So finally I tried the following lines and it solved my problem:

sudo apt-get clean
sudo mv /var/lib/apt/lists /var/lib/apt/lists.broke
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get update