diff --git a/tardis/plasma/properties/nlte_rate_equation_solver.py b/tardis/plasma/properties/nlte_rate_equation_solver.py
index 7b46e43a7ed..5a7655886d6 100644
--- a/tardis/plasma/properties/nlte_rate_equation_solver.py
+++ b/tardis/plasma/properties/nlte_rate_equation_solver.py
@@ -94,7 +94,7 @@ def calculate(
         )
         
         raise NotImplementedError("NLTE ionization hasn't been fully implemented yet!") 
-        
+
     @staticmethod
     def calculate_rate_matrix(
         phi_shell,
@@ -399,7 +399,7 @@ def prepare_ion_recomb_coefficients_nlte_ion(
                 level_population_fraction.loc[coll_ion_coeff.index]
                 * coll_ion_coeff
             )
-            .groupby(level=(0, 1))
+            .groupby(level=("atomic_number", "ion_number"))
             .sum()
         )
         total_coll_recomb_coefficients = (