How can I work around Type.IsAssignableFrom not working with inherited interfaces?

public interface IParentInterface
{

}

public interface IChildInterface : IParentInterface
{

}

public class ParentClass<T> where T: IParentInterface
{

}

public class ChildClass : ParentClass<IChildInterface>
{

}

public class TestClass
{
    public ChildClass Property { get; set; }
}

[TestFixture]
public class ScratchPad
{

    [Test]
    public void Assignabl()
    {
        var tc = new TestClass();
        var tct = tc.GetType();

        var pi = tct.GetProperty("Property");

        Assert.IsNotNull(pi);

        Assert.IsTrue(typeof(ParentClass<IChildInterface>).IsAssignableFrom(pi.PropertyType));
        Assert.IsTrue(typeof(ParentClass<>).IsAssignableFrom(pi.PropertyType));
        Assert.IsTrue(typeof(ParentClass<IParentInterface>).IsAssignableFrom(pi.PropertyType));

    }

}

public class ParentClass<ParentInterface>

public class ChildClass : ParentClass<ChildInterface>

public class ParentClass<T>

interface IParent { }
interface IChild : IParent { }

Parent<IChild>

Parent<IParent>

List<ParentInterface> parentList = List<ParentInterface>();
List<ChildInterface> childList = parentList;

public interface ParentInterface2 : ChildInterface {}

childList.Add(new ParentInterface2Implementation());