Mike, sorry to reply so quickly to my email, but I realized I could have been clearer. Please replace: self._data_source["ParticleMassMsun"].sum(dtype='float64') with self._data_source.quantities["TotalQuantity"]("ParticleMassMsun") in both cases. -- Stephen Skory s at skory.us http://stephenskory.com/ 510.621.3687 (google voice)