Matching error with FlannBased vial

I am using OpenCV2.2 on Windows 7. What I'm trying to do is to detect a specific object in another image using this code:

// Read the two image files
Mat image1 = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
Mat image2 = imread( argv[2], CV_LOAD_IMAGE_GRAYSCALE );

Mat finalImage = imread(argv[2]);

if( !image1.data || !image2.data ) {
    std::cout << " --(!) Error reading images " << std::endl;
    return -10;
}

//Construct the SURF Detector
int minHessian = 400;
SurfFeatureDetector detector( minHessian );

//Extract the keypoints for each image
std::vector<KeyPoint> keypoints1, keypoints2;
detector.detect(image1,keypoints1);
detector.detect(image2,keypoints2);


//Calculate descriptors (feature vectors)
SurfDescriptorExtractor extractor;
Mat descriptors1, descriptors2;
extractor.compute(image1,keypoints1,descriptors1);
extractor.compute(image2,keypoints2,descriptors2);

//Define the Matcher
FlannBasedMatcher matcher;

std::cout << "matcher constructed!" << std::endl;

std::vector<vector<DMatch >> matches;
matcher.knnMatch(descriptors1, descriptors2, matches, 2);

std::cout << "matches: " << matches.size() << std::endl;

std::vector<DMatch > good_matches;

    //THIS LOOP IS SENSITIVE TO SEGFAULTS
for (int i = 0; i < min(descriptors2.rows-1,(int) matches.size()); i++) 
    {
        if((matches[i][0].distance < 0.8*(matches[i][1].distance)) && ((int) matches[i].size()<=2 && (int) matches[i].size()>0))
        {
            good_matches.push_back(matches[i][0]);
        }
    }

std::cout << "good_matches: " << good_matches.size() << std::endl;

VS2010 builds and compiles the code without errors. My problem is that in some (and not all) cases where execution jumps to a line

matcher.knnMatch(descriptors1, descriptors2, matches, 2);

cmd stops and returns a message like: "Assertion Failed ( dataset.Type() == CvType(T)::type()) in an unknown function, etc. etc., ending before the line flann.hpp 105"

This happens when there is no similarity between the images (and not for all cases), although the descriptors (necessary for matching) are correctly extracted from both images. The code works fine if I use BruteForce.

I also tried the code from OpenCV:

http://opencv.itseez.com/doc/tutorials/features2d/feature_homography/feature_homography.html

. , , Opencv.

std::vector< DMatch > matches;

matcher.match( descriptors_object, descriptors_scene, matches );

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