diff --git a/tardis/plasma/properties/nlte_rate_equation_solver.py b/tardis/plasma/properties/nlte_rate_equation_solver.py
index 2ef015b32ed..d37c4464679 100644
--- a/tardis/plasma/properties/nlte_rate_equation_solver.py
+++ b/tardis/plasma/properties/nlte_rate_equation_solver.py
@@ -18,7 +18,7 @@
 NLTE_POPULATION_NEGATIVE_POPULATION_TOLERANCE = (
     -1e-10
 )  # Maximum negative population allowed before solver fails
-NLTE_POPULATION_SOLVER_CHARGE_CONSERVATION_TOLERANCE = 1e-6 # Arbitrary tolerance for charge conservation, should be changed to a more reasonable value
+NLTE_POPULATION_SOLVER_CHARGE_CONSERVATION_TOLERANCE = 1e-6  # Arbitrary tolerance for charge conservation, should be changed to a more reasonable value
 
 
 class NLTEPopulationSolverRoot(ProcessingPlasmaProperty):
@@ -150,18 +150,18 @@ def calculate(
                 solution.x[-1],
                 number_density[shell],
                 ion_numbers,
-            ) 
+            )
             # Check that the electron density is still in line with charge conservation
             # after removing negative populations
             assert (
                 np.abs(
                     np.sum(ion_numbers * ion_number_density[shell])
                     - electron_densities[shell]
-                ) / electron_densities[shell]
+                )
+                / electron_densities[shell]
                 < NLTE_POPULATION_SOLVER_CHARGE_CONSERVATION_TOLERANCE
             ), "Charge conservation not fulfilled after correcting for negative populations, solver failed."
 
-
         # TODO: change the jacobian and rate matrix to use shell id and get coefficients from the attribute of the class.
 
         return ion_number_density, electron_densities
diff --git a/tardis/plasma/tests/test_nlte_solver.py b/tardis/plasma/tests/test_nlte_solver.py
index 8a20187c529..6379e7a3e0a 100644
--- a/tardis/plasma/tests/test_nlte_solver.py
+++ b/tardis/plasma/tests/test_nlte_solver.py
@@ -3,7 +3,10 @@
 
 import pytest
 from numpy.testing import assert_allclose
-from tardis.plasma.properties import NLTEPopulationSolverRoot, NLTEPopulationSolverLU
+from tardis.plasma.properties import (
+    NLTEPopulationSolverRoot,
+    NLTEPopulationSolverLU,
+)
 from tardis.plasma.properties.nlte_rate_equation_solver import (
     calculate_jacobian_matrix,
     calculate_rate_matrix,