csencrypt.exe failing with "An unexpected error occurred"

Windows 8.1, Azure 2.2 SDK.

I'm trying to use csencrypt.exe to encrypt a new password for Azure RDP access, as follows:

C:\Program Files\Microsoft SDKs\Windows Azure\.NET SDK\v2.2\bin>csencrypt.exe encrypt-password -Thumbprint MYTHUMBPRINT

This launches the program and prompts me for a password. However, after entering and confirming the password, it throws a very generic error message (likely an InvalidOperationException?):

An unexpected error occurred: Operation is not valid due to the current state of the object.

I've got the cert with that thumbprint installed under Current User -> Personal. The Windows event log shows nothing. What could be going wrong here?