Operations under detectMultiScales, need help

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For some reason, while running my code, it seems that all operations below detectMultiScales stopped working. When I remove the detectMultiScales line in my code, the program works very well. BTW, I am using Visual Studio 2013 Community and here is my code

#include "stdafx.h"
#include <string>
#include <iostream>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/objdetect/objdetect.hpp>

using namespace std;
using namespace cv;

Mat cam;
VideoCapture camera(0);
CascadeClassifier car_cascade;

void detectAndDisplay(Mat frame) {
    std::vector<Rect> cars;
    Mat frame_gray;
    cvtColor(frame, frame_gray, CV_BGR2GRAY);
    equalizeHist(frame_gray, frame_gray);
    car_cascade.detectMultiScale(frame_gray, cars, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(30, 30));
    for (int i = 0; i < cars.size(); i++) {
        Point center(cars[i].x + cars[i].width*0.5, cars[i].y + cars[i].height*0.5);
        ellipse(frame, center, Size(cars[i].height*0.5, cars[i].width*0.5), 0, 0, 360, Scalar(0, 255, 0));
    }
    imshow("Frame", frame);
}

int main()
{
    if (!car_cascade.load("C:\\Users\\---\\Documents\\cascade.xml")) {
        return -1;
    }

    if (!camera.isOpened()) {
        printf("Error: cannot open a camera\n");
    }

    while (true)
    {
        if (camera.isOpened()) {
            camera >> cam;
        }

        if (!cam.empty()) {
            detectAndDisplay(cam);
        }

        if (cv::waitKey(1) == 27) {
            break;
        }

    }

    return 0;

}