OpenCV reading RGB Values. Values are not consistent for every run - c++

Question

When running this code with the same image and mask I occasionally get values that differ previously. For example
Normal/Expected Results
Abnormal Results
As can be seen in the two results I somehow receive negative RGB values. The Abnormal Results show up around every 5-8 runs for the exact same image with the exact same mask.
What do I need to change to prevent the occasional abnormal result?
#include <opencv2/opencv.hpp>
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string>
using namespace cv;
using namespace std;
float get_color(Mat img,Mat &mask){
Mat img1 = img;
Mat hist;
int dims = 1;
int histSize = 255;
float hranges[] = { 0, 255 };
const float *ranges = {hranges};
calcHist(&img1,1,0,mask,hist, dims, &histSize, &ranges ,true ,false);
int sum=0;
for(int i = 0;i<256;i++){
sum += hist.at<float>(i,0);
}
Mat weights = hist/sum;
float hist_avg=0.0;
for(int i = 0;i<256;i++){
hist_avg += i*weights.at<float>(i,0);
}
return hist_avg;
}
int main(int argc, char** argv){
Mat aa = imread("/Users/dnguyen/Desktop/snapshot207042/pic.jpg", CV_LOAD_IMAGE_COLOR);
vector<Mat> bgr;
split(aa, bgr);
Mat b = bgr[0];
Mat g = bgr[1];
Mat r = bgr[2];
for(unsigned int i=1;i<23;i++){
stringstream ss;
ss << i;
string str = ss.str();
string file_name = "/Users/dnguyen/Desktop/okay/data/card_masks/"+str+"_mask.png";
Mat mask = imread(file_name,0);
Mat cc;
threshold(mask,cc,90,255,THRESH_BINARY);
float b_avg = get_color(b, cc);
float g_avg = get_color(g, cc);
float r_avg = get_color(r, cc);
cout << b_avg << ","<< g_avg << "," << r_avg << endl;
}
}