C++ OpenCV 4.4.0 solvePnP error: (-215:Assertion failed)

Question

I am trying to write a simple augmented reality program that, given and output video and the camera intrinsic parameters, projects 3 lines into each video frame. These lines are the x,y and z axis.

I'm asking for help not for the program itself but for the solvePnP function, because I just don´t understand what I am doing wront to get this error:

terminate called after throwing an instance of 'cv::Exception' what(): OpenCV(4.4.0) /home/nel/opencv-4.4.0/modules/calib3d/src/solvepnp.cpp:753: error: (-215:Assertion failed) ( (npoints >= 4) || (npoints == 3 && flags == SOLVEPNP_ITERATIVE && useExtrinsicGuess) ) && npoints == std::max(ipoints.checkVector(2, CV_32F), ipoints.checkVector(2, CV_64F)) in function 'solvePnPGeneric'

This is my code:

#include <vector>
#include <opencv2/highgui/highgui_c.h>


using namespace cv;
using namespace std;

int main(int, char**)
{
    VideoCapture cap("./video.mp4"); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;

    Mat image;
    Size patternsize(9,6); //interior number of corners
    vector<Point2f> corners;
   vector<Point3f> axis={Point3f(0.0,0.0,0.0), Point3f(0.0,0.1,0.0),Point3f(0.0,0.0,0.1), Point3f(0.1,0.0,0.0)};
 
FileStorage fs("intrinsics.yml", FileStorage::READ);
    if (!fs.isOpened())
    {
      cerr << "failed to open " << "intrinsic.yml" << endl;
      return 0;
    }

     Mat camera_matrix, distortion_coefficients;
    cout << "reading R and T" << endl;
    fs["camera_matrix"] >> camera_matrix;
    fs["distortion_coefficients"] >> distortion_coefficients;

  fs.release();

   
    for(;;)
    {
        Mat frame;
        Mat rvec, tvec, points2d;
        cap >> frame; // get a new frame from camera
        cvtColor(frame, image, COLOR_BGR2GRAY);


        bool patternfound = findChessboardCorners(image, patternsize, corners,
        CALIB_CB_ADAPTIVE_THRESH + CALIB_CB_NORMALIZE_IMAGE
        + CALIB_CB_FAST_CHECK);

        if(patternfound) cornerSubPix(image, corners, Size(11, 11), Size(-1, -1),TermCriteria(CV_TERMCRIT_EPS + CV_TERMCRIT_ITER, 30, 0.1));
        else{cerr<<"Error: pattern not found"<<endl;return 0;}

   cv::solvePnP(axis, corners, camera_matrix, distortion_coefficients, rvec, tvec); 
       
  cout<<"Extrinsic params calculated"<<endl;


  cv::projectPoints(axis, rvec, tvec, camera_matrix, distortion_coefficients, points2d);

  MatIterator_<double> it, end;
for( it = points2d.begin<double>(), end = points2d.end<double>(); it != end; ++it)
{
    cv::line(image, (Point)points2d.at<double>(0,0), (Point)*it, (255,0,0), 3,8);
}
        imshow("image", image);
        if(waitKey(30) >= 0) break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}