Getting DefaultValue for optional Guid through reflection?

I have the following code:

    public static void MethodWithOptionalGuid(Guid id = default(Guid)) { }

    public static void MethodWithOptionalInteger(int id = 2) { }

    public static void MethodWithOptionalString(string id = "33344aaa") { }

    public static void MethodWithoutOptionalParameter(int id, Guid longId) { }

    static void Main(string[] args)
    {
        var methods = typeof(Program).GetMethods(BindingFlags.Public | BindingFlags.Static).ToList();

        foreach (var method in methods)
        {
            PrintMethodDetails(method);
        }

        Console.ReadLine();
    }

    static void PrintMethodDetails(MethodInfo method)
    {
        Console.WriteLine(method.Name);

        foreach (var parameter in method.GetParameters().ToList())
        {
            Console.WriteLine(parameter.Name +
                              " of type " + parameter.ParameterType.ToString() +
                              " with default value:" + parameter.DefaultValue);
        }

        Console.WriteLine();
    }

It prints the following:

MethodWithOptionalGuid
id of type System.Guid with default value:

MethodWithOptionalInteger
id of type System.Int32 with default value:2

MethodWithOptionalString
id of type System.String with default value:33344aaa

MethodWithoutOptionalParameter
id of type System.Int32 with default value:
longId of type System.Guid with default value:

My question is: why the Guid's default value is not recognized?

I expected to receive something like "0000000-..." . I also tried initializing the optional parameter with new Guid() and same result. I tried as well other structs, like TimeSpan and the behavior is the same.

I expected that all value types would behave the same (as seen in integer example).