Using the Region of Interest in regionprops

trying to use the regionprops function with the 'UseROI' to tell me when a person passes through a region of interest. However it gives me an error when i run the code saying there are inputs missing. detector = vision.PeopleDetector('ClassificationModel', 'UprightPeople_96x48', 'ClassificationThreshold', 3,'UseROI',true); bbox = step(detector, im); im2 = insertObjectAnnotation(im, 'rectangle', bbox, 'person');

the error occurs in the bbox step saying Not enough input arguments; expected 2 (in addition to the object handle), got 1. does anyone know what the inputs that are missing would have to be in order to get this working?