Error sending Slack notification to private channel from appveyor

I tried to set up Slash notification to my private channel for Appveyor using the following configuration (xxxxxx is my token encrypted here):

notifications:
  - provider: Slack
    channel: dev
    auth_token:
      secure: xxxxxx

My Appveyor build show me the folloing error:

Error sending Slack notification: Error sending Slack message: channel_not_found

When testing the Slash api with the following URL, it works: https://slack.com/api/chat.postMessage?token=xxxxxx&channel=dev&text=hello&pretty=1 (where xxxxxx is my token without encryption).

It also works if the channel is not private.

Solution