Overlay : filesystem not supported on lowerdir

I tried to mount an external storage device (a usb flash drive) with overlay (as the lowerdir the usb and as upper a temporary ramdisk) mkdir /data mkdir /data/lower mkdir /data/rw mkdir /data/root Then creating the ramdisk mount -t tmpfs none /data/rw mkdir /data/rw/upper mkdir /data/rw/work I mounted the usb device mount -t vfat /dev/sdb1 /data/lower To finally use overlay : mount -t overlay -o lowerdir=/data/lower,upperdir=/data/rw/upper,workdi=/data/rw/work none /data/root Which give me the folowwing error > wrong fs type, bad option, bad superblock on overlay, missing codepage or helper program, or other error > overlayfs: filesystem on '/data/lower' not supported But according to the documentation > The lower filesystem can be any filesystem supported by Linux and does not need to be writable Is there a link with automount and if so how I can fix it ?