Why this code gives me floating point Error?

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;

int main(int argc, char** argv)
{
    Mat src, dst;
    src=imread("/home/chris/Pictures/1.png",1);
    if(!src.data){return -1;}

    vector<Mat> bgr_planes;
    split(src, bgr_planes);

    int histSize=256;
    float range[]={0,256};
    const float* histRange={range};

    bool uniform=true; bool accumulate=false;
    Mat b_hist, g_hist, r_hist;

    calcHist(&bgr_planes[0], 1, 0, Mat(), b_hist, 1, &histSize, &histRange, uniform, accumulate);
    calcHist(&bgr_planes[1], 1, 0, Mat(), b_hist, 1, &histSize, &histRange, uniform, accumulate);
    calcHist(&bgr_planes[2], 1, 0, Mat(), b_hist, 1, &histSize, &histRange, uniform, accumulate);

    int hist_w=512; int hist_h=400;
    int bin_w=cvRound((double) hist_w/histSize);
    Mat histImage(hist_h, hist_w, CV_8UC3, Scalar(0,0,0));

    normalize(b_hist, b_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat());
    normalize(g_hist, g_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat());
    normalize(r_hist, r_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat());

    for(int i=0; i<histSize; i++)
    {
        line(histImage, Point(bin_w*(i-1), hist_h- cvRound(b_hist.at<float>(i-1))),
        Point(bin_w*(i),hist_h - cvRound(b_hist.at<float>(i))),
        Scalar(255,0,0),2,8,0);
        line(histImage, Point(bin_w*(i-1), hist_h- cvRound(g_hist.at<float>(i-1))),
        Point(bin_w*(i),hist_h - cvRound(g_hist.at<float>(i))),
        Scalar(0,255,0),2,8,0);
        line(histImage, Point(bin_w*(i-1), hist_h- cvRound(r_hist.at<float>(i-1))),
        Point(bin_w*(i),hist_h - cvRound(r_hist.at<float>(i))),
        Scalar(0,0,255),2,8,0);

    }
    cvNamedWindow("Demo", CV_WINDOW_AUTOSIZE);
    imshow("Demo",histImage );
    waitKey(0);

    return 0;
}