title: Library NuGet configuration is invalid
body: <p>VS2015 Update 3.</p>
  
  <p>I created a plain .net core class library.
  Right-click on Project->References-> Manage Nuget packages
  throws the following error?</p>
  
  <p>What is missing?</p>
  
  <p><a href="https://i.stack.imgur.com/ELVX4.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ELVX4.png" alt="enter image description here"></a></p>