Comment 1 for bug 834174

Same issue for me.
Reason: missing /lib/x86_64-linux-gnu/libnss-files* in ramdisk (i'm on a 64bits system)
Adding correct cp in /usr/share/initramfs-tools/scripts/init-premount/dropbear is a work-around
Dropbear scripts and docs not up to date, many other files are missing (/etc/shadow, /etc/nsswitch.conf, /etc/group...)