Cannot mount encrypted partitions on boot up

I'm trying to encrypt the /home partition. I've been successful in doing this by following the instructions here:

https://help.ubuntu.com/community/EncryptedFilesystemHowto

I have the following lines in the following files:

/etc/crypttab:
home /dev/sda3

/etc/fstab
/dev/mapper/home /chome reiserfs defaults 0 1

When I boot up I get 'Special device /dev/mapper/home does not exist.'

I have to manually type 'cryptsetup create home /dev/sda3' each time I boot up to get into the encrypted partition.

I expect to be prompted for the password on boot but I just get that error, what am I doing wrong?