How do I solve an error when running dense optical flow on an image pyramid in OpenCV?

When I create an image pyramid from each of the two images and try to perform an optical flow using the calcOpticalFlowFarneback method, I get the following error.

OpenCV(3.4.3) Error: Assertion failed (0 <= i && i < (int)v.size()) in cv::debug_build_guard::InputArray::getMat, file
D:\opencv343\sources\modules\core\src\matrix_wrap.cpp, line 88 OpenCV:
terminate handler is called! The last OpenCV error is: OpenCV(3.4.3)
Error: Assertion failed (0 <= i && i < (int)v.size()) in
cv::debug_build_guard::InputArray::getMat, file
D:\opencv343\sources\modules\core\src\matrix_wrap.cpp, line 88

Code

// The header file only includes iostream, vector, opencv2.hpp and uses std and cv namespaces
    #include "header.h"


    void tracking(const Mat& inOldImg, const Mat& inNewImg) {
        const int maxLevel = 5;
        vector<Mat> oldImgPyr;
        buildOpticalFlowPyramid(inOldImg, oldImgPyr, Size(3, 3), maxLevel, false);

        vector<Mat> newImgPyr;
        buildOpticalFlowPyramid(inNewImg, newImgPyr, Size(3, 3), maxLevel, false);
    
        Mat flow(inOldImg.size(), CV_32FC2);
        calcOpticalFlowFarneback(oldImgPyr, newImgPyr, flow, 0.25, maxLevel, 15, 3, 5, 1.2, 0);
        return;
    }

    int main() {
        string oldImgPath = "old.bmp";
        string newImgPath = "new.bmp";

        Mat oldImg = imread(oldImgPath, 0);
        Mat newImg = imread(newImgPath, 0);

        tracking(oldImg, newImg);
    
        return 0;
    }

What I tried.

• Flow vectors were saved in flow when run with a single image instead of an image pyramid.
• The same error occurred when the scale parameter was set to 0.5.
• The same error occurred when 0 was specified as the level of the pyramid.
• I updated my OpenCV version to 3.4.13 and still had the same problem.

Environment

    OpenCV: 3.4.3
    VisualStudio: 2019

Update

Referring to the OpenCV reference, calcOpticalFlowFarneback requires an 8-bit single-channel input image, while calcOpticalFlowPyrLK requires either an 8-bit single-channel image or an image pyramid. Does this mean that calcOpticalFlowFarneback internally converts the image to a pyramid image?

Description of the input and output of calcOpticalFlowFarneback in the reference

    prev    first 8-bit single-channel input image.
    next    second input image of the same size and the same type as prev.

Description of the input and output of calcOpticalFlowPyrLK in the reference

    prevImg first 8-bit input image or pyramid constructed by buildOpticalFlowPyramid.
    nextImg second input image or pyramid of the same size and the same type as prevImg.