Luks, ssh unlock, Strange behaviour, Invalid authorized_keys file

I don't know why, but I can't manage to follow these instructions on my debian stable. After installing dropbear and busybox, I tried to run initramfs -u. I got here a strange warning: # update-initramfs -u update-initramfs: Generating /boot/initrd.img-4.9.0-4-amd64 dropbear: WARNING: Invalid authorized_keys file, remote unlocking of cryptroot via SSH won't work! Then, I tried to look in the file /etc/initramfs-tools/root/.ssh/id_rsa, but there is no folder root in the initramfs folder. I also tried to run dropbearkey -t dss -f /etc/initramfs-tools/etc/dropbear/dropbear_dss_host_key but there is no folder /etc/initramfs-tools/etc/, so this command also fails. Of course, I can create these folders, but I'm afraid that this strange behaviour does not come from a deeper error. If it can help, here is the content of the initramfs folder: me@server:/etc/initramfs-tools# ls conf.d hooks initramfs.conf modules scripts update-initramfs.conf Thank you!