Massprop works on the area enclosed by a closed polygon. If you have holes in the section, what I do is to open the hole and the outer boundary about 1/10,000 of a unit wide and add line segments joining the areas, then making sure the polygon is closed. The section properties of the microscopic sliver of material you added is too small to matter. You can relocate to the centroid if needed (Massprop gives you the coordinates) but it is not really needed.