Can't get web content from on-premise http GitHub Server in Power Query

In a Power Query query, I am using Web.Contents to get the contents from on on-premise GitHub server using a query that works fine in PostMan, Microsoft Edge, and Google Chrome (no auth), but when used in Power Query for an Power BI dashboard, I get this error "DataSource.Error: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
Details: http://github.myserver.com/myuser.atom.
Any suggestions?  Same results with http and https.

= Xml.Document(Web.Contents("http://github.myserver.com/myuser.atom"))
Thanks,
Michael