Interop Prob w/ LPArray - COM Client throwing exception

I would appreciate any help/insight that anyone can give on this problem.
I have a legacy app that uses the following IDL interface definition:

interface IMyInterface2 : IUnknown
{
 HRESULT doit( [in, size_is(msgLength)]UCHAR* msg, [in]ULONG
msgLength );
};

When my legacy COM Client calls my .Net component that implements this
interface, an access violation (0xC0000005) exception is thrown. I've
tried everything and can not figure out why the exception is thrown in the
COM Client.

Here is the COM Client:
#include "stdafx.h"
#include "IMyInterface2_h.h"
#include "IMyInterface2_i.c"

int _tmain(int argc, _TCHAR* argv[])
{
 HRESULT hr = S_OK;

        hr = ::CoInitialize( NULL );

 CLSID rcid = {0};
 rcid.Data1 = 0x5F850215; rcid.Data2 = 0x7AC2; rcid.Data3 = 0x48b8;
 rcid.Data4[0] = 0xA8; rcid.Data4[1] = 0xCF; rcid.Data4[2] = 0xA0;
 rcid.Data4[3] = 0x08; rcid.Data4[4] = 0xF7; rcid.Data4[5] = 0x33;
 rcid.Data4[6] = 0xD6; rcid.Data4[7] = 0x61;

 CComPtr<IMyInterface2> iMyInterface2;

        hr = iMyInterface2.CoCreateInstance( rcid, NULL,
CLSCTX_INPROC_SERVER );

 UCHAR* pBytes = (UCHAR*)::CoTaskMemAlloc( 20 );

 ::ZeroMemory( pBytes, 20 );

 iMyInterface2->doit( pBytes, 20 );

 ::CoUninitialize();

 return 0;
}

And here is the .Net service component. The .Net component must be copied
in the same directory as the COM client.

using System;
using System.Runtime.InteropServices;

namespace ComSrv
{
 [GuidAttribute("5F850215-7AC2-48b8-A8CF-A008F733D661")]
 public class MyClass : IMyInterface2
 {
  public void doit( byte[] msg, ulong msgLength )
  {
  }
 }

 [ComImport]
 [GuidAttribute("B55AFAEB-BC7D-4ca9-8065-F9A45FE378B6")]
 [InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]
 public interface IMyInterface2
 {
  void doit(
   [In, MarshalAs( UnmanagedType.LPArray,
SizeParamIndex=1, ArraySubType=UnmanagedType.U1 )]byte[] msg,
   [In]ulong msgLength );
 }
}

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