关于OpenCV的问题

CvVideoWriter* m_pVideoWriter;
CvSize nSize;
nSize.width=654
nSize.height=494;double fps=25;
is_color=0;m_pVideoWriter = cvCreateVideoWriter("output.avi",
CV_FOURCC('D','I','V','3'),
fps,
nSize,
is_color); 这段执行之后，那个m_pVideoWriter应该是个不为0的值。我现在是0
请问高手们，错在哪里？谢谢