after sudo apt upgrade:

pi @ raspberrypi: ~ $ sudo apt update
Old: 1 http://raspbian.raspberrypi.org/raspbian stretch InRelease
Reading package lists ... Done
Building a dependency tree
Read status information ... Done
24 packages can be upgraded. Run 'apt list --upgradable' to see them.
pi @ raspberrypi: ~ $ sudo apt upgrade
Reading package lists ... Done
Building a dependency tree
Read status information ... Done
Calculation of updates ... Done
The following packages will be updated:
  ca-certificates dpkg dpkg-dev file libcups2 libcupsimage2 libdpkg-perl libexiv2-14 libfaad2 libgs9 libgs9-common libmagic-mgc libmagic1 libpam-systemd
  libsoup-gnome2.4-1 libsoup2.4-1 libsystemd0 libudev1 patch shared-mime-info systemd systemd-sysv tzdata udev
24 updated, 0 newly installed, 0 removed and 0 not upgraded.
Necessary to download 0 B / 18.8 MB of archives.
After this operation 15.4 kB of disk space will be additionally used.
Continue? [Y / n] t
Reading change logs ... Done
Pre-configuration of packages ...
dpkg: error: the "fgets" function returned an empty string from "/var / lib / dpkg / triggers / File"
E: Sub-process / usr / bin / dpkg returned an error code (2)