opencv的null pointer function cvGetMat

想要把工业ccd截屏处理，用opencv，但是截图的时候就弹出如题的错误,希望可以早点得到解决方法，在线等
代码如下： 
    IplImage* pImg=NULL;
    pImg = cvCreateImageHeader(cvSize(m_nMaxWid,m_nMaxHei),IPL_DEPTH_8U,3);
    cvSetData(pImg,m_pImageBuffer,m_nMaxWid*3); 
    //memcpy(pImg->imageData,m_pImageBuffer,m_nMaxWid*m_nMaxHei*3);
    CvCapture* cap=cvCreateCameraCapture(-1);
    char filename[100];
    static int count=0;
    cvNamedWindow("Camera",CV_WINDOW_AUTOSIZE);
    pImg=cvQueryFrame(cap);
    sprintf(filename,"%s%d%s","image",count++,".jpg");  
    cvShowImage("Camera",pImg);
    //cvWaitKey(30000);
    cvSaveImage(filename,pImg);
    cvDestroyAllWindows();
    cvReleaseImage(&pImg);
    cvReleaseCapture(&cap);
    if (NULL != pImg) {
        delete []pImg;///English:
    }
pImg = cvCreateImageHeader(cvSize(m_nMaxWid,m_nMaxHei),IPL_DEPTH_8U,3);
cvSetData(pImg,m_pImageBuffer,m_nMaxWid*3);
再加上句cvcopy(pimage0,plmg);就可以了