diff --git a/examples/fitting_simulated_data.py b/examples/fitting_simulated_data.py
index f0d4383..89cb93c 100644
--- a/examples/fitting_simulated_data.py
+++ b/examples/fitting_simulated_data.py
@@ -67,7 +67,7 @@
 srm_model = MatrixModel(
     matrix=srm, input_axis=SpectralAxis(ph_energies), output_axis=SpectralAxis(ph_energies), c=1 * u.ct / u.ph
 )
-srm_model.inputs_units = {"x": u.ph}
+srm_model.input_units = {"x": u.ph}
 
 with quantity_support():
     plt.figure()
diff --git a/sunkit_spex/models/instrument_response.py b/sunkit_spex/models/instrument_response.py
index 7dfe138..06ab59e 100644
--- a/sunkit_spex/models/instrument_response.py
+++ b/sunkit_spex/models/instrument_response.py
@@ -10,6 +10,7 @@ class MatrixModel(Fittable1DModel):
     c = Parameter(fixed=True)
 
     def __init__(self, matrix, input_axis, output_axis, c):
+        self._input_units = None
         self.inputs_axis = input_axis
         self.output_axis = output_axis
         self.matrix = matrix
@@ -22,11 +23,11 @@ def evaluate(self, x, c):
 
     @property
     def input_units(self):
-        return self.inputs_units
+        return self._input_units
 
     @input_units.setter
     def input_units(self, units):
-        self.inputs_units = units
+        self._input_units = units
 
     @staticmethod
     def _parameter_units_for_data_units(inputs_unit, outputs_unit):