'Invalid Key' error from Paramiko

I'm trying to set up Fabric for deploying my Python web application and Paramiko开发者_开发知识库 is barfing on my private RSA key. I had been using my key successfully for 6 months, so I know it's good. In case having a passphrase was the problem, I just made a new key with no passphrase and still get the error. Help?

I don't know if this helps you but here's how I set my SSH user/key in fabric.

env.user = "username"
env.key_filename = "/path/to/ssh/keyfile"

And it seems to work fine.