Debian :: Encrypt Partitions On A Usb Stick Using Squeeze?

I am trying to encrypt partitions on a usb stick using squeeze. I will be installing Arch on that stick, but as I am using Debian as the OS to do the encrypting I thought here was the best place to ask. I've not had any dealings with encryption before so i'm a little lost. It's not working as I would expect basically - here is the error mssge :

spoovy@peony ~ $ sudo cryptsetup -c aes-xts-plain -y -s 512 luksFormat /dev/sdb5
WARNING!
This will overwrite data on /dev/sdb5 irrevocably.
Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase:
Verify passphrase:
device-mapper: reload ioctl failed: Invalid argument
Failed to setup dm-crypt key mapping for device /dev/sdb5.

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