How to get correct depth value using kinect? I get all 0

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Hey,

I'm new to opencv and all computer vision stuff. I'm trying to get the depth of one specific pixel point from the kinect image. I get the code how to get un-encoded depth from the topic /kinect2/hd/image_depth_rect

  void depthcb(const sensor_msgs::ImageConstPtr& msg)
{
        cv_bridge::CvImageConstPtr depth_img_cv;
        cv::Mat depth_mat;
        // Get the ROS image to openCV
        depth_img_cv = cv_bridge::toCvCopy(msg, sensor_msgs::image_encodings::TYPE_16UC1);
        // Convert the uints to floats
        depth_img_cv->image.convertTo(depth_mat, CV_32F, 0.001);

        float depth = depth_img_cv->image.at<float>(10,10);

        ROS_INFO("Depth: %f", depth);

}

int main(int argc, char* argv[])
{
    ros::init(argc, argv, "depth");
    ros::NodeHandle nh;
    ros::Subscriber sub = nh.subscribe("/kinect2/hd/image_depth_rect", 1000, depthcb);
    ros::spin();
    return 0;
}

But I get all 0 output for the depth, and I tried quite few different pixels. If I understand this right, the pixel coordinate should be within the range (1080,1920) for kinect v2 ?

I'm pretty sure the kinect is working fine since when I echo kinect2/hd/image_depth_rect I get nonzeros and the output image looks right to me. Any idea what I did wrong?

Many thanks!