Proper unit should be 1/(m^3), not m^3. Although the mistake is small and non-critical, Maxima not need it.
patch for physical_constants.mac
Changed units to 1/m^3 as suggested. Committed as ab86cb42e5e85ae6c90c06ab35cd72e646c5f259 in Git. Thanks to Andrew for pointing out the problem. Closing this report as fixed.