Assign not working if parameters not explicitly set to ref

Hi,

I've got the following bit of code. Where assign does not work

namespace UnitTests
{

    public static class MyClass
    {
        public static void Method(String param)
        {
            param = "not set";
        }
    }

    public class Caller
    {

        public void Call()
        {
            String b = "bye";

            MyClass.Method(b);
        }

    }

    [TestFixture]
    public class tests
    {
        [Test]
        public void test()
        {
            Mock myMock = MockManager.Mock(typeof(MyClass), Constructor.StaticNotMocked);
            myMock.ExpectCall("Method").Args(new Assign("hello"));

            Caller a = new Caller();
            a.Call();
        }
    }
}

If I make the parameter param explicitly a ref then it works. Eg. When the code is

namespace UnitTests
{

    public static class MyClass
    {
        public static void Method(ref String param)
        {
            param = "not set";
        }
    }

    public class Caller
    {

        public void Call()
        {
            String b = "bye";

            MyClass.Method(ref b);
        }

    }

    [TestFixture]
    public class tests
    {
        [Test]
        public void test()
        {
            Mock myMock = MockManager.Mock(typeof(MyClass), Constructor.StaticNotMocked);
            myMock.ExpectCall("Method").Args(new Assign("hello"));

            Caller a = new Caller();
            a.Call();
        }
    }
}

Anyone know why you have to explicitly set ref on the parameters if using assign, as I thought that all reference objects should be passed as reference anyway?