stylus renders blank file when --watching

I'm just setting up stylus on a simple project and am running this command

stylus --watch --compress --out css/ src/styl/

and it works fine the first time, and it does realize when I've updated a source file but it saves a blank file in the destination.

Any ideas?

I'm using node v0.6.21 - I've tried with and without --compress if that makes any difference

EDIT

I know you can write the commands in various ways, but I don't think it is that.

stylus -w src/styl/ -o css/

also has the same problem