diff --git a/imports/energy_systems/xlsx_heat_pump_parameters.py b/imports/energy_systems/xlsx_heat_pump_parameters.py
index 33f45b02..5e6269d0 100644
--- a/imports/energy_systems/xlsx_heat_pump_parameters.py
+++ b/imports/energy_systems/xlsx_heat_pump_parameters.py
@@ -122,7 +122,7 @@ class XlsxHeatPumpParameters:
     popt, _ = curve_fit(self._objective_function, [x_values, out_temp], heat_pump_data)
     return popt.tolist()
 
-  def _objective_function(self, xdata, a1, a2, a3, a4, a5, a6):
+  def _objective_function(self, xdata, a1, a2, a3, a4, a5, a6) -> float:
     """
     Objective function for computing coefficients
     :param xdata: