Debug Assertion Failed, with SetTimer

Hi,

I got this error when call the SetTimer (..) for the 2nd time:

Debug Assertion Failed

File: wincore.cpp
Line: 311

Basically, I call SetTimer

void CChildDlg: OnStart( )
{

  // do something
   ...
   SetTimer(100, 500, NULL);

}

void CChildDlg: OnEnd( )
{
    KillTimer(100);
    ...
   // do something
}

Click Start Button, timer running OK, then click End to stop timer,
then click Start button, and I get the Debug Assertion Failed.

When debug, it points to:
BOOL CWnd::Attach(HWND hWndNew)
{
    ASSERT(m_hWnd == NULL); // only attach once, detach on destroy
    ASSERT(FromHandlePermanent(hWndNew) == NULL);
        // must not already be in permanent map

    if (hWndNew == NULL)
        return FALSE;

    CHandleMap* pMap = afxMapHWND(TRUE); // create map if not exist
    ASSERT(pMap != NULL);

    pMap->SetPermanent(m_hWnd = hWndNew, this);

#ifndef _AFX_NO_OCC_SUPPORT
    AttachControlSite(pMap);
#endif

    return TRUE;
}

Can anyone help ?

I changed to Release Version, also same issue