diff --git a/beast/physicsmodel/grid_and_prior_weights.py b/beast/physicsmodel/grid_and_prior_weights.py
index e7d7bffd..5105d7cb 100644
--- a/beast/physicsmodel/grid_and_prior_weights.py
+++ b/beast/physicsmodel/grid_and_prior_weights.py
@@ -174,17 +174,17 @@ def compute_age_mass_metallicity_weights(
 
             # compute the mass weights
             if len(aindxs) > 1:
-                # cur_masses = _tgrid_single_age["M_ini"]
-                # deal with repeat masses - happens for MegaBEAST
-                cur_masses = np.unique(_tgrid_single_age["M_ini"])
-                umass_grid_weights = compute_mass_grid_weights(cur_masses)
                 if isinstance(mass_prior_model, dict):
                     mass_prior = PriorMassModel(mass_prior_model)
                 else:
                     mass_prior = mass_prior_model
-                umass_prior_weights = mass_prior(cur_masses)
+
+                # deal with repeat masses - happens for MegaBEAST
+                cur_masses = np.unique(_tgrid_single_age["M_ini"])
                 n_masses = len(_tgrid_single_age["M_ini"])
                 if len(cur_masses) < n_masses:
+                    umass_grid_weights = compute_mass_grid_weights(cur_masses)
+                    umass_prior_weights = mass_prior(cur_masses)
                     mass_grid_weights = np.zeros(n_masses, dtype=float)
                     mass_prior_weights = np.zeros(n_masses, dtype=float)
                     for k, cmass in enumerate(cur_masses):
@@ -192,8 +192,9 @@ def compute_age_mass_metallicity_weights(
                         mass_grid_weights[gvals] = umass_grid_weights[k]
                         mass_prior_weights[gvals] = umass_prior_weights[k]
                 else:
-                    mass_grid_weights = umass_grid_weights
-                    mass_prior_weights = umass_prior_weights
+                    cur_masses = _tgrid_single_age["M_ini"]
+                    mass_grid_weights = compute_mass_grid_weights(cur_masses)
+                    mass_prior_weights = mass_prior(cur_masses)
             else:
                 # must be a single mass for this age,z combination
                 # set mass weight to zero to remove this point from the grid