missing dependencies Microsoft.CodeAnalysis.CSharp.Scripting

While trying to install the C# scripting package Microsoft.CodeAnalysis.CSharp.Scripting from nuget, but was getting odd messages about missing dependencies. I spent ages trying to figure it out as all the dependency versions appeared to match (e.g.

Unable to find a version of ‘Microsoft.CodeAnalysis.Common’ that is compatible with ‘Microsoft.CodeAnalysis.CSharp 2.0.0 constraint: Microsoft.CodeAnalysis.Common (= 2.0.0)’, ‘Microsoft.CodeAnalysis.Scripting.Common 2.0.0 constraint: Microsoft.CodeAnalysis.Common (= 2.0.0)’

however, eventually I discovered that you neeed .NET 4.6. Doh!