Empty message error with Clickatell

Hello,

I'm getting the following error when using VBO -> SMS Bulk -> SMS Framework -> Clickatell:

Sending SMS to [phone number] failed. The gateway said ERR: 107, Empty message

The SMS Framework + Clickatell are themsleves correctly set up, as I was able to send an SMS earlier on (before I found the SMS Bulk module), but I haven't been able to get SMS Bulk to work successfully.

I'm using:

- Views Bulk Operations (VBO) 6.x-1.10
- SMS Bulk 6.x-2.0
- SMS Framework 6.x-1.0

I'm happy to supply any other info about my setup if that helps track down the issue.