Create account key not valid

[itniels]

I am using Certes 3.0.3 via LinqPad to create a new account, and it does return a key, but when I later use the key to create a new order, it fails with the following message:

One or more errors occurred. (Fail to load resource from 'https://acme-v02.api.letsencrypt.org/acme/new-acct'.urn:ietf:params:acme:error:accountDoesNotExist: No account exists with the provided key)