mean_age_population returns NaN when w has entries equal to zero

>>> cm = mpm.compadre.fetch("COMPADRE")
>>> m = cm.get_from_id(248193)
>>> m
MPM(
  S = [[0.43, 0.  , 0.  , 0.  , 0.  , 0.  ],
       [0.29, 0.44, 0.  , 0.  , 0.  , 0.  ],
       [0.01, 0.42, 0.69, 0.  , 0.  , 0.  ],
       [0.  , 0.  , 0.22, 0.22, 0.  , 0.  ],
       [0.  , 0.  , 0.  , 0.  , 0.85, 0.  ],
       [0.  , 0.  , 0.  , 0.71, 0.06, 0.85]],
  F = [[0.  , 0.  , 0.82, 1.34, 1.93, 2.37],
       [0.  , 0.  , 0.  , 0.  , 0.  , 0.  ],
       [0.  , 0.  , 0.  , 0.  , 0.  , 0.  ],
       [0.  , 0.  , 0.  , 0.  , 0.  , 0.  ],
       [0.  , 0.  , 0.  , 0.  , 0.  , 0.  ],
       [0.  , 0.  , 0.  , 0.  , 0.  , 0.  ]],
 ...
)
>>> m.mean_age_population
nan

This is because mean_age_population is implemented as

self._mean_age_population = self.mean_age_class @ self.w

and that when w_i, the mean age in class i is nan -- as should be the case. However, since this class does not contribute to the asymptotic constitution of the population, the fact that the mean age is not defined in it should not matter.