Multiple BitBlts Eventually Crash. Why?

I'm calling the following function in a timer. It draws a specific bitmap
from an array of bitmaps (animation approach)

After a few hundred calls I get an unhandled exception 0xC0000005 Access
Violation. When I try to debug it drops me on the line
"memDC.SelectObject(pOldBitmap) ;" and I see that pOldBitmap is 0x00000000.
What's going on?

    - Mark

CBitmap m_bitmapArray[MAX_BITMAPS] ; // is defined in the class
the argument "nRef" is the reference to an array of bitmaps

///////////////////////////////////////////////////////////////////////////////
void CMyCtrl::ShowBitmapInRect (CDC *pDC, CRect rectTarget, int nRef)
{
    if ((rectTarget.Width() != m_rectDraw.Width()) &&
        (rectTarget.Height() != m_rectDraw.Height()))
        return ;

    if ((nRef < 0) || (nRef >= m_nSteps))
        return ;

    // make memory dc
    CDC memDC ;
    memDC.CreateCompatibleDC(pDC) ;

    // grab the bitmap
    CBitmap *pOldBitmap = memDC.SelectObject(&m_bitmapArray[nRef]) ;

    // draw the memory dc
    pDC->BitBlt(rectTarget.left, rectTarget.top,
    rectTarget.Width(), rectTarget.Height(),
    &memDC, 0, 0, SRCCOPY) ;

    // release the bitmap
    memDC.SelectObject(pOldBitmap) ;

} //ShowBitmapInRect