Incorrect filename extension

In the Terminal whole doing sudo apt-get update I am often getting the message

ignoring 'get deb.list.bck in directory '/etc/apt/sources.list.d' as it has invalid filename extension

What is the correct extension?

Check for the file ended with deb.list.bck in /etc/apt/sources.list.d directory.If you found any then move it to another directory(Home).

It allow only the files ended with .list, .list.save, .list.bak.

OR

Try this simple command to move all the files ended with .bck in /etc/apt/sources.list.d directory to your ~/Desktop directory.

sudo find /etc/apt/sources.list.d/ -type f -name "*.bck" -exec mv {} ~/Desktop \;