vector subscript out of range?

[15900854096]

ERROR:
vector classes_fg;
classes_fg.reserve(keypoints_fg.size());
for (size_t i = 0; i < keypoints_fg.size(); i++)
{
classes_fg.push_back(i);
}
Mat descs_fg;
Mat descs_bg;
descriptor->compute(im_gray, keypoints_fg, descs_fg);
descriptor->compute(im_gray, keypoints_bg, descs_bg);
RIGHT:
Mat descs_fg;
Mat descs_bg;
descriptor->compute(im_gray, keypoints_fg, descs_fg);
descriptor->compute(im_gray, keypoints_bg, descs_bg);

vector<int> classes_fg;
classes_fg.reserve(keypoints_fg.size());
for (size_t i = 0; i < keypoints_fg.size(); i++)
{
	classes_fg.push_back(i);
}

this is my solution,maybe help for you