_queryDescriptors.type() == trainDescType [closed]

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Error message: terminate called after throwing an instance of 'cv::Exception'

what(): OpenCV(4.1.2-dev) /home/ztftrue/Downloads/opencv/modules/features2d/src/matchers.cpp:756: error: (-215:Assertion failed) _queryDescriptors.type() == trainDescType in function 'knnMatchImpl'

Code:

Mat input = cv::imread("input.jpg",IMREAD_COLOR);
cout << "input type: "<<input.type() << endl;
cout << "descriptors type: " << descriptors.type() << endl;
bowide.compute(input, keypoints, descriptors);

Out put:

input type: 21 descriptors type: 0

I try:

input.convertTo(input, CV_32F, 1 / 255.0);

or

input.convertTo(input, CV_32F);

and filter keypoint vector

      if (!keypoints.empty() && keypoints.size() > 10) {
            cv::Mat descriptors;
        extractor->compute(it, keypoints, descriptors);
        if (!descriptors.empty()) {
            descriptors.convertTo(descriptors, CV_32F);
            trainingDescriptors.push_back(descriptors);
        } else {
            std::cout << "- No descriptors found." << std::endl;
            }
    } else {
        std::cout << "- No keypoints found." << std::endl;
    }