The world coordinates can be more than two-dimensional -- for example, (lat, lon, spec), or it could even be (lat, spec).  So the correct solution might be to create a zeroed array for all of the dimensions, fill in `xy.lonangle.degree` in index `self.wcs.lng` and `xy.latangle.degree` in index `self.wcs.lat`.  Alternatively, you could check that the world coordinate dimensions are 2 (`self.wcs.naxis == 2`) and that the wcs.lat and wsc.lng are 0 and 1 (in either order), and if not, raise an exception.