WNDPROC and incorrect parameter error

Friends,

I am a newbie to Win32 programming and I am mucking around with a silly
issue due to my lackness in win32 knowledge. Basically, I am trying to
create a child window inside an main window. The main window gets created
properly, in the main windows WM_CREATE message I am calling a method which
creates a child window, here is what my WNDPROC for main window looks like,

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM
lParam)
{
       switch (message)
       {
             case WM_CREATE:
                  CreateChildWindow ( hWnd, WndProc3, _T("helper_clas") );
                  break;
             case WM_DESTROY:
                  PostQuitMessage(0);
                  break;
             default:
                  return DefWindowProc(hWnd, message, wParam, lParam);
     }
}

In the CreateChildWindow function I am creating a child window,

VOID CreateChildWindow ( HWND hWnd, WNDPROC wndProc, LPCWSTR className )
{
         WNDCLASSEX wcx ={0};
         HWND hChild;

         wcx.cbSize = sizeof(WNDCLASSEX);
         wcx.lpszClassName = className;
         wcx.hInstance = hInst;
         wcx.lpfnWndProc = wndProc;
         wcx.hbrBackground = (HBRUSH)::GetStockObject(GRAY_BRUSH);
         wcx.hCursor = LoadCursor(NULL, IDC_ARROW);
         wcx.hIcon = LoadIcon(NULL, IDI_APPLICATION);
         wcx.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

         RegisterClassEx(&wcx);

         hChild = CreateWindowEx(0L, className, NULL, WS_VISIBLE | WS_CHILD
, 45,0,200,300, hWnd, NULL, NULL, NULL);

         if ( hChild == NULL )
              MessageBox ( hWnd, _T( "No luck" ), _T("E"), MB_OK );
}

Window not gets created and the GetLastError shows something like the
parameter is incorrect (error number: 87). I am not sure what parameter went
wrong over here. Please help me.

Thanks,
Vadi