DebuggerDisplay on generic class

I have a problem applying the DebuggerDisplay attribute on a generic class:

[DebuggerDisplay("--foo--")]
class Foo
{
}

[DebuggerDisplay("Bar: {t}")]
class Bar<T>
{
    public T t;
}

When inspecting an object of type Bar<Foo> I would expect it to show as Bar: --foo--, but I get Bar: {Foo}

What am I doing wrong?