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SURF feature detector detects no keypoints

Greetings

This is opencv 4.1.0 on windows 10;

using the below simple image SURF feature detector gives no keypoints. code snippet:

std::vector<cv::KeyPoint> keypoints;
cv::Ptr<cv::xfeatures2d::SURF> detector = cv::xfeatures2d::SURF::create( 1 );
detector->detect( img, keypoints);
printf("keypoints.size=%zu\n", keypoints.size());

keypoints.size() is 0, any explanation?

img:

image description

Any hints?