Why does Reflecting on own types not work in T4 template?

I have a .NET Core 2.1 ASP.NET MVC application in which I have a design time T4 template which I want to use to generate some code. Currently, it looks like this:

<#@ template debug="false" hostspecific="false" language="C#" #>
<#@ assembly name="System.Core" #>
<#@ assembly name="$(SolutionDir)\Bar\bin\Debug\netcoreapp2.1\Bar.dll" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Text" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="Bar" #>
<#@ output extension=".txt" #>
<#
    var myType = typeof(string);
    var myName = y.Name;
    Write(myName);    
#>

This works: the .txt file generated by this template contains "String".

However, now I have a class Foo in Foo.cs in the same folder as my template:

namespace Bar
{
    public class Foo
    {
    }
}

Now I want to use Foo instead of string, so I just change the line

var myType = typeof(string);

to

var myType = typeof(Foo);

and save the template. Now I get an error:

Running transformation: 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 stack trace is as follows:

File name: 'System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
    at Microsoft.VisualStudio.TextTemplating9418B77BB4C607966DB4F31F6C9D57D97D9892B08324572567CEAC228ECCD2BE3839F9E2A0AE0ECA2D5DD0CCC161A70762C40A6D67A8DC505F3E88E86A36C7CE.GeneratedTextTransformation.TransformText()
    at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)
    at CallSite.Target(Closure , CallSite , Object )   
    at Microsoft.VisualStudio.TextTemplating.TransformationRunner.PerformTransformation()

Why does it throw this error? Why does reflection work for the built-in string type, but somehow depend on .NET Framework 4.2.1 for my own .NET Core 2.1 type?