Problem creating an MDI interface

//First this function is called to register the windows

inline bool	WinObject::RegisterWindows(WNDPROC FrameProc, WNDPROC ChildProc)
{
	WNDCLASSEX Win;

	Win.cbSize		= sizeof(WNDCLASSEX); 
	Win.style		= CS_HREDRAW | CS_VREDRAW;
	Win.lpfnWndProc		= FrameProc;
	Win.cbClsExtra		= 0;
	Win.cbWndExtra		= 0;
	Win.hInstance		= hInstance;
	Win.hIcon		= LoadIcon(hInstance, (LPCTSTR)IDI_MSCYOUNG);
	Win.hCursor		= LoadCursor(NULL, IDC_ARROW);
	Win.hbrBackground	= (HBRUSH)(COLOR_WINDOW+1);
	Win.lpszMenuName	= (LPCTSTR)IDC_MSCYOUNG;
	Win.lpszClassName	= FrameClass;
	Win.hIconSm		= LoadIcon(hInstance, (LPCTSTR)IDI_SMALL);

	if(RegisterClassEx(&Win)==0) return FALSE;

  Win.lpfnWndProc    = ChildProc; 
  Win.hIcon          = LoadIcon(hInstance, (LPCTSTR)IDI_CHILDICON); 
  Win.hIconSm	   = LoadIcon(hInstance, (LPCTSTR)IDI_CHILDICON);
  Win.lpszMenuName   = (LPCTSTR) NULL; 
  Win.cbWndExtra     = 0; 
  Win.lpszClassName  = ChildClass; 

  if(RegisterClassEx(&Win)==0) return FALSE;

	return TRUE;
}

//Then this function is called to create the frame window

inline HWND	WinObject::CreateMainWin()
{
HWND  MainWin;
MainWin = CreateWindow(  // This call to CreateWindow returns 0
  FrameClass,
  AppTitleEX,
  WS_OVERLAPPEDWINDOW,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  CW_USEDEFAULT,
  NULL,
  NULL,
  hInstance,
  NULL
);

DWORD Oops = GetLastError();  // This gives error code 1400 (ERROR_INVALID_WINDOW_HANDLE)

return MainWin;
}

LRESULT CALLBACK MDIProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
int wmId, wmEvent;

switch (message) 
 {
  case WM_CREATE: 
    CLIENTCREATESTRUCT  ccs; 
 
    ccs.hWindowMenu   = GetSubMenu(GetMenu(FrameHwnd), WINDOWMENU); 
    ccs.idFirstChild  = MDI_CHILD;

    ClientHwnd = CreateWindow("MDICLIENT"
                          ,NULL
                          ,WS_CHILD | WS_CLIPCHILDREN | WS_VSCROLL | WS_HSCROLL
                          ,0,0,0,0
                          ,FrameHwnd
                          ,NULL
                          ,hInstance
                          ,&ccs);

    ShowWindow(ClientHwnd,SW_SHOW);
    break;

  case WM_COMMAND: 
    wmId    = LOWORD(wParam); 
    wmEvent = HIWORD(wParam); 
    switch (wmId)
    {
      <lots of stuff here that isnt important>
      default:
        return DefFrameProc(hWnd, ClientHwnd, WM_COMMAND, wParam, lParam);
    }
    break;
  case WM_DESTROY: 
    PostQuitMessage(0);
    break;
  default: 
    return DefFrameProc(hWnd, ClientHwnd, WM_COMMAND, wParam, lParam);
  }
  return 0;
}