Can't Step into Native Code

Calling C++ from C#. Method runs OK, but I can't step into it to debug. It shows the disassembly screen and says source code not available. Any ideas?

Code below, in case it helps...

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
	class Program
	{
		[return: MarshalAs(UnmanagedType.I4)]
		[DllImport("ClassLibrary1.dll", SetLastError = true)]
		static extern int DoSomething();
		static void Main(string[] args)
		{
			int x = DoSomething();
			Console.WriteLine(x.ToString());
			Console.ReadKey();
		}
	}
}

#pragma once

using namespace System;

namespace ClassLibrary1
{
	__declspec(dllexport) int DoSomething();
}
// This is the main DLL file.

#include "stdafx.h"

#include "ClassLibrary1.h"

int DoSomething()
{
	return 3;
}