Cannot create MDI Client Win

I'm trying to make an MDI application, but my code to create the client window keeps failing. GetLastError() returns 1406 (Cannot create a top level child window). As far as I can tell, I'm doing exactly what The Forger does to create the client window (and the only difference between that and MSDN is using CreateWindowEx instead of CreateWindow):

Code:

 CLIENTCREATESTRUCT ccs; 
 
  // Retrieve the handle to the window menu and assign the 
  // first child window identifier. 
  ccs.hWindowMenu = GetSubMenu(GetMenu(m_hwnd), IDX_MDIMENU); 
  ccs.idFirstChild = 0; 
 
  // Create the MDI client window. 
 
  m_hwndMDIClient = CreateWindowEx(WS_EX_CLIENTEDGE, "mdiclient", NULL,
        WS_CHILD | WS_CLIPCHILDREN | WS_VSCROLL | WS_HSCROLL | WS_VISIBLE,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        m_hwnd, (HMENU)IDC_MAIN_MDI, GetModuleHandle(NULL), (LPVOID)&ccs);

  if(!m_hwndMDIClient)
  {
    char temp[50];
    MessageBox(NULL,"Could not create MDI Client window!",itoa(GetLastError(),temp,10),MB_OK);
    return NULL;
  }

I'm obviously doing something wrong, but I have no idea how I should change it

Wow took me a while but I figured out the problem. I was setting m_hwnd (a class member variable) to the return value of CreateWindow for my frame window. Of course, since this code was executing inside the WM_CREATE command for the frame window, CreateWindow hadn't returned yet and m_hwnd was invalid.