Coda: "Could not import ssh key. The file is not in a supported format"

After I added a passphrase to my existing SSH key, Coda started showing me this error:

Could not import SSH key. The file is not in a supported format.

The solution is to enter a passphrase to that key into the password input. This was confusing to me because I thought it was asking for the password of an actual user I'm trying to connect as.