apt – Problem unlinking the file auxfiles

When i get apt update on a Debian 9 server, i get following error

# apt update
Get:1 http://ftp.debian.org/debian stretch-updates InRelease [91.0 kB]
Hit:2 http://security.debian.org stretch/updates InRelease                                                                          
Hit:3 https://packages.sury.org/php stretch InRelease                              
Ign:4 http://ftp.us.debian.org/debian stable InRelease                             
Hit:5 http://ftp.us.debian.org/debian stable Release
Fetched 91.0 kB in 0s (165 kB/s)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
All packages are up to date.
W: Problem unlinking the file auxfiles - Clean (21: Is a directory)
# 

The problem is fixed by deleting

rm -rf /var/lib/apt/lists/