Puttygen passphrase not working

I generated a public/private SSH key pair (with no passphrase) using Puttygen on a Windows machine, and copied the public key up to my Linux server. When I use the private key in Putty everything works as expected. However, when I try to ssh from the Windows command line, I get prompted for a passphrase. Since there isn't one, I can't login.ssh -T myserver.com Enter passphrase for key 'C:\Users\myusername\.ssh\private.ppk': Permission denied (publickey, gssapi-with-mic, password).