T4 Template failing - Could not load file or assembly System.Runtime

I'm trying out Rider because I'm having issues with VS. I'm really liking it but I'm having an issue with a T4 template. It works in VS but in Rider I'm getting

Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
  at JetBrains.Rider.TextTemplating.GeneratedTextTransformation.<TransformText>g__GetParentProjectItem|1_1(<>c__DisplayClass1_0& )
  at JetBrains.Rider.TextTemplating.GeneratedTextTransformation.TransformText()
  at JetBrains.Rider.TextTemplating.ReSharperGeneratedTransformationExecutor.<>c__DisplayClass1_0.<PostRegisterMain>b__0(Lifetime lifetime)
  at JetBrains.Lifetimes.Lifetime.UsingNested[T](Func`2 action)
  at JetBrains.Rider.TextTemplating.ReSharperGeneratedTransformationExecutor.PostRegisterMain(String[] args)
  at JetBrains.Rider.TextTemplating.ReSharperGeneratedTransformationExecutor.Main(String[] args)

Process finished with exit code -532,462,766.

This is the start of the template

<#@ template language="C#" debug="true" hostspecific="true" #>

<#@ output extension=".cs" #>
<#@ assembly name="EnvDTE" #>
<#@ assembly name="System.Core" #>
<#@ assembly name="System.Linq" #>
<#@ assembly name="System.Runtime" #>
<#@ import namespace="System" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="System.IO" #>
<#@ import namespace="EnvDTE" #>

If I don't reference System.Runtime then I get

The type 'IEnumerable' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.