diff --git a/rocketpy/mathutils/function.py b/rocketpy/mathutils/function.py
index 190d38d25..c8d6470e6 100644
--- a/rocketpy/mathutils/function.py
+++ b/rocketpy/mathutils/function.py
@@ -75,21 +75,22 @@ def __init__(
         -------
         None
         """
+        # Set input and output
+        if inputs is None:
+            inputs = ["Scalar"]
+        if outputs is None:
+            outputs = ["Scalar"]
+
         inputs, outputs, interpolation, extrapolation = self._check_user_input(
             source, inputs, outputs, interpolation, extrapolation
         )
+
         # initialize variables to avoid errors when being called by other methods
         self.get_value_opt = None
         self.__polynomial_coefficients__ = None
         self.__akima_coefficients__ = None
         self.__spline_coefficients__ = None
 
-        # Set input and output
-        if inputs is None:
-            inputs = ["Scalar"]
-        if outputs is None:
-            outputs = ["Scalar"]
-
         # store variables
         self.set_inputs(inputs)
         self.set_outputs(outputs)