I ran into the same problem. It was discovered that it was a link to the package, below, in my .csproj file. Most likely, the recent link to the package you added.
< PackageReference Include = "Microsoft.AspNetCore.Server.Kestrel.Https" = "1.1.2" >
, ...
< PackageReference Include = "Microsoft.AspNetCore.Server.Kestrel.Https" = "1.1.2" / >