Assert Debug Fail:GetTextExtentPoint32(m_hAttribDC, lpszString, nCount, &size))

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MFC

My program is to display some points with their positions(x,y) on a graph. When I use mouse to drag any point, its position will automatically changed. Updated position is implemented following this code(using thread):

m_thread =AfxBeginThread((AFX_THREADPROC)MainThread,this)
UINT CAtwWnd::MainThread(LPVOID pParam)
{
CAtwWnd *pMainDlg = (CAtwWnd*)pParam;

static SChartXYPoint pPoint;
TCHAR strTemp[32]={0,};

   while(1)
   {
        pMainDlg->m_chart.EnableRefresh(false);
        pMainDlg->InitGraph1();

        wsprintf(strTemp, _T("[%d](%d,%d)"), (int)index,(int)xPoint,(int) yPoint);
        pBalloon[index]->SetLabelText(strTemp);
        pBalloon[index] = pMainDlg->m_pPointSeries->CreateBalloonLabel(index, strTemp);

        pBalloon[index]->SetVisisble(true);
        pMainDlg->m_pPointSeries->SetVisible(true);
        pMainDlg->m_chart.EnableRefresh(true);
        pMainDlg->SetAtwGraphStep(1);
    }
return 0;
}

Mean while:

void CChartLabel<PointType>::SetLabelText(const TChartString& strText)  
{  
   m_strLabelText = strText; 
   m_pParentCtrl->RefreshCtrlAtw();
}

And:

void CChartCtrl::RefreshCtrlAtw()
{
// Window is not created yet, so skip the refresh.
if (!GetSafeHwnd())
    return;
if (m_iEnableRefresh < 1)
{
    m_bPendingRefresh = true;
    return;
}

// Retrieve the client rect and initialize the
// plotting rect
CClientDC dc(this) ;  
CRect ClientRect;
GetClientRect(&ClientRect);
m_PlottingRect = ClientRect;        

// If the backgroundDC was not created yet, create it (it
// is used to avoid flickering).
if (!m_BackgroundDC.GetSafeHdc() )
{
    CBitmap memBitmap;
    m_BackgroundDC.CreateCompatibleDC(&dc) ;
    memBitmap.CreateCompatibleBitmap(&dc, ClientRect.Width(),ClientRect.Height()) ;
    m_BackgroundDC.SelectObject(&memBitmap) ;
}

// Draw the chart background, which is not part of
// the DrawChart function (to avoid a background when
// printing).
DrawBackground(&m_BackgroundDC, ClientRect);
ClientRect.DeflateRect(3,3);
DrawChart(&m_BackgroundDC,ClientRect);
for (int i=0; i<4 ;i++)
{
    if (m_pAxes[i])
        m_pAxes[i]->UpdateScrollBarPos();
}

Invalidate();
}

when dragging points on graph I gets these errors somtimes: Debug Assertion Failed ( afxwin1.inl, line 639, and 646)

_AFXWIN_INLINE CSize CDC::GetTextExtent(LPCTSTR lpszString, int nCount) const
{
    ASSERT(m_hAttribDC != NULL);
    SIZE size;
    VERIFY(::GetTextExtentPoint32(m_hAttribDC, lpszString, nCount, &size));
    return size;
}
_AFXWIN_INLINE CSize CDC::GetTextExtent(const CString& str) const
{
    ASSERT(m_hAttribDC != NULL);
    SIZE size;
    VERIFY(::GetTextExtentPoint32(m_hAttribDC, str, (int)str.GetLength(), &size));
    return size;
}

Could you help me to fix this problem? I tried to find some ways to fix but doesn't work. :(