[ubuntu] Problem loading encrypted volume on boot (16.04.01)

I'm a serial distro-hopper and I'm finally trying out Ubuntu 16.04.01.

I keep the same /home partition no matter what distro I use.

Typically, what I do when I try out a new distribution is the following:

1. Find the UUID of the device (/dev/sdb1)
2. Add a line to /etc/crypttab:

data UUID=(UUID of /dev/sdb1) none luks

3. Add a line to /etc/fstab:

/dev/mapper/data /home ext4 auto 0 2

--

Now, the weird thing is, I'm never prompted for a password for dev/sdb1!

I've checked and doublechecked to ensure that I'm using the correct UUID -- that is, of the block device and not the mapper.

Can anyone help?