Octopus Deploy and Proget

This weekend I switched my local Octopus Deploy server to use the ProGet as the package repository.

Generally speaking, it was pretty painless switch - but I was getting errors until I added an advanced MSBuild Argument (/p:OctoPackPublishApiKey=BuildKey) on to provide an API Key; obviously you need to configure this in ProGet :)
Initially I didn't think I would need to provide this API Key as the user the build agent was running under a user account that had full access to the feeds; it seems, however, that this is not the case when you are running normal authentication (i.e. not domain joined).