OpenCV Bag of Words: Assertion failed (!_descriptors.empty()) - c++

Question

I'm trying to create Bag-Of-Words so I can train the SVM later. I'm new with OpenCV so I used a code that I found on the Internet. The problem is that I have the following error:
OpenCV Error: Assertion failed (!_descriptors.empty()) in add, file /build/buildd/opencv-2.4.8+dfsg1/modules/features2d/src/bagofwords.cpp, line 57
terminate called after throwing an instance of 'cv::Exception'
what(): /build/buildd/opencv-2.4.8+dfsg1/modules/features2d/src/bagofwords.cpp:57: error: (-215) !_descriptors.empty() in function add
Aborted (core dumped)
And here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <opencv2/opencv.hpp>
#include <fstream>
#include <iostream>
#include <string>
#include <dirent.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
using namespace cv;
using namespace std;
int main(int argc, char** argv)
{
Ptr<FeatureDetector> features = FeatureDetector::create("FAST");
Ptr<DescriptorExtractor> descriptors = DescriptorExtractor::create("FAST");
Ptr<DescriptorMatcher> matcher = DescriptorMatcher::create("FlannBased");
//defining terms for bowkmeans trainer
TermCriteria tc(TermCriteria::MAX_ITER + TermCriteria::EPS, 10, 0.001);
int dictionarySize = 1000;
int retries = 1;
int flags = KMEANS_PP_CENTERS;
BOWKMeansTrainer bow_trainer(dictionarySize, tc, retries, flags);
BOWImgDescriptorExtractor bowDE(descriptors, matcher);
//training data now
Mat mat_features1, mat_features2;
Mat img = imread("../positive_images/2014-12-07 19-55-07 804ea51d (1).JPG", 0);
Mat img2 = imread("../positive_images/Pictures23.bmp_0000_0342_0104_0564_0543.png", 0);
vector<KeyPoint> keypoints, keypoints2;
features->detect(img, keypoints);
features->detect(img2,keypoints2);
descriptors->compute(img, keypoints, mat_features1);
descriptors->compute(img2, keypoints2, mat_features2);
bow_trainer.add(mat_features1);
bow_trainer.add(mat_features2);
Mat dictionary = bow_trainer.cluster();
bowDE.setVocabulary(dictionary);
return 0;
}
And here is how the code is compiled:
g++ BOW_creator.cpp -o BOW_creator `pkg-config --cflags --libs opencv`
Do you know what the problem could be? Can you tell me how to fix it?
Let me know if you need me to provide more information.