VS2015 C# interactive: error CS7069: Reference to type 'Object' claims it is defined in 'System.Runtime', but it could not be found

Question

I just updated to VS2015 Update 2, and started playing around with the C# interactive window. I wanted to use a static method in a static class in one of my .NET 4.0 targete

Answer 1

What ultimately fixed it for me was entering this right in the C# Interactive window.:

#r "System.Runtime"

If there's anyone that can provide a thorough background explanation as to why this worked, I'd love to give you the accepted answer. I just got lucky.