OPENCV MAT内存访问错误

我正在尝试编写一个函数，以补偿openCV项目中的镜头渐渐。问题在于，当我尝试访问垫子中的第一个像素时，我会收到一个内存访问错误。这可能是一个愚蠢的错误，但是如果您可以向我指出，我将不胜感激。这是功能：

void EckOpCam::FlatField(cv::Mat & parmI)
{
// workaround for access violation error
cv::Mat I = parmI;
const int channels = I.channels();
if (flatField.rows == 0)
    MakeFlatFieldMat(I.rows);
// accept only char type matrices
ASSERT(I.depth() == CV_8U);
// make sure the matrices are the same size
ASSERT(I.cols == flatField.cols);
ASSERT(channels == flatField.channels());

// Try the really slow way...
int nRows = I.rows;
int nCols = I.cols;
unsigned int ipix0, ipix1, ipix2;   // B,G,r values of pixel in I
float fpix0, fpix1,fpix2;           // B,G,r values of pixel in flatField
float pix0, pix1, pix2;
switch (channels)
{
case 1:
    for (int y = 0; y < nRows; ++y)
        for (int x = 0; x < nCols; ++x)
        {
            {
                ipix0 = I.at<cv::Vec3b>(x, y)[0];
                fpix0 = flatField.at<cv::Vec3f>(x, y)[0];
                pix0 = ipix0 * fpix0;
                I.at<cv::Vec3b>(x, y)[0] = (pix0 > 255) ? 255 : uchar(pix0);
            }
        }
    break;
case 3:
    for (int y = 0; y < nRows; ++y)
        for (int x = 0; x < nCols; ++x)
        {
            {
                ipix0 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[0]); //<<< memory access error!
                ipix1 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[1]);
                ipix2 = static_cast<uchar>(I.at<cv::Vec3b>(x, y)[2]);
                fpix0 = flatField.at<cv::Vec3f>(x, y)[0];
                fpix1 = flatField.at<cv::Vec3f>(x, y)[1];
                fpix2 = flatField.at<cv::Vec3f>(x, y)[2];
                pix0 = ipix0 * fpix0;
                pix1 = ipix1 * fpix1;
                pix2 = ipix2 * fpix2;
                I.at<cv::Vec3b>(x, y)[0] = (pix0 > 255) ? 255 : uchar(pix0);
                I.at<cv::Vec3b>(x, y)[1] = (pix1 > 255) ? 255 : uchar(pix1);
                I.at<cv::Vec3b>(x, y)[2] = (pix2 > 255) ? 255 : uchar(pix2);
            }
        }
}

}

错误是：异常抛出：读取访问违规。cv :: vec ::操作员返回0x240c37ec4a8。

给出的地址正是i.data的地址。怎么了？感谢您的帮助。