copy_exec command not found when executing update-initramfs

I'm currently trying to include cryptsetup into my initramfs so i can boot after having encrypted the root partition. I created a hook-script in

/usr/share/initramfs-tools/hooks/my_hook

With the following content copy_exec /sbin/cryptsetup /sbin But everytime i try to execute (in chroot) update-initramfs -u -k all It fails with: root@ubuntu update-initramfs -u -k all /usr/share/initramfs-tools/hooks/my_hook :1 /usr/share/initramfs- tools/hooks/my_hook copy_exec: not found E: /usr/share/initramfs-tools/hooks/my_hook failed with return 127