为什么出现错误"The D3D device has a non-zero reference count"?

我使用DX9, DXUT框架。
我定义一个简单的类，
.h文件：
=================================================================

class CMyMesh01_One

{

public:

CMyMesh01_One(void);

~CMyMesh01_One(void);

public:

bool LoadMeshFromFile(char filename[], LPDIRECT3DDEVICE9 & pD3DDevice);

public:

LPDIRECT3DVERTEXDECLARATION9 m_pVD;

struct stctVertex

{

float x, y, z;

DWORD color;

};

};

=================================================================
.cpp文件：
=================================================================

#include "dxstdafx.h"

#include "MyMesh01_One.h"



CMyMesh01_One::CMyMesh01_One(void)

{m_pVD = NULL;}



CMyMesh01_One::~CMyMesh01_One(void)

{

if (m_pVD)

{

m_pVD->Release();

m_pVD = NULL;

}

}

bool CMyMesh01_One::LoadMeshFromFile(char filename[], LPDIRECT3DDEVICE9 & pD3DDevice)

{

//Vd

D3DVERTEXELEMENT9 decl[] = {

{0, 0, D3DDECLTYPE_FLOAT3, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_POSITION, 0},

{0, 12, D3DDECLTYPE_D3DCOLOR, D3DDECLMETHOD_DEFAULT, D3DDECLUSAGE_COLOR, 0},

D3DDECL_END()

};



HRESULT hr;

V(hr = pD3DDevice->CreateVertexDeclaration(decl, &m_pVD));



return true;

}

================================================================================
之后我在主程序中声明
CMyMesh01_One g_clsMyMesh01_One;
并且在OnCreateDevice中调用

HRESULT CALLBACK OnCreateDevice( IDirect3DDevice9* pd3dDevice, const D3DSURFACE_DESC* pBackBufferSurfaceDesc, void* pUserContext )

{

g_clsMyMesh01_One.LoadMeshFromFile("C:\\test.txt", pd3dDevice);

return S_OK;

}

但关闭程序时提示The D3D device has a non-zero reference count, meaning some objects were not released.
为什么产生这样的错误呀？