diff --git a/src/ampform/dynamics/builder.py b/src/ampform/dynamics/builder.py
index 6c169def9..25ef061a6 100644
--- a/src/ampform/dynamics/builder.py
+++ b/src/ampform/dynamics/builder.py
@@ -100,7 +100,7 @@ def create_non_dynamic_with_ff(
     )
     return (
         form_factor,
-        {meson_radius: sp.S.One},
+        {meson_radius: 1},
     )
 
 
@@ -205,7 +205,7 @@ def __energy_dependent_breit_wigner(
         parameter_defaults = {
             res_mass: resonance.mass,
             res_width: resonance.width,
-            meson_radius: sp.S.One,
+            meson_radius: 1,
         }
         return breit_wigner_expr, parameter_defaults
 
@@ -228,9 +228,9 @@ def __create_form_factor(
             meson_radius=meson_radius,
         )
         parameter_defaults = {
-            meson_radius: sp.S.One,
+            meson_radius: 1,
         }
-        return form_factor, parameter_defaults
+        return form_factor, parameter_defaults  # type: ignore[return-value]
 
     @staticmethod
     def __create_symbols(
diff --git a/tests/helicity/test_helicity.py b/tests/helicity/test_helicity.py
index 562ac7c20..30bada78e 100644
--- a/tests/helicity/test_helicity.py
+++ b/tests/helicity/test_helicity.py
@@ -13,6 +13,7 @@
 from ampform.helicity import (
     HelicityAmplitudeBuilder,
     HelicityModel,
+    ParameterValue,
     ParameterValues,
     _generate_kinematic_variables,
     formulate_wigner_d,
@@ -170,6 +171,14 @@ def test_use_helicity_couplings(self, reaction: ReactionInfo):
 
 
 class TestHelicityModel:
+    def test_parameter_defaults_item_types(
+        self, amplitude_model: tuple[str, HelicityModel]
+    ):
+        _, model = amplitude_model
+        for symbol, value in model.parameter_defaults.items():
+            assert isinstance(symbol, sp.Symbol)
+            assert isinstance(value, ParameterValue.__args__)  # type: ignore[attr-defined]
+
     def test_rename_symbols_no_renames(
         self, amplitude_model: tuple[str, HelicityModel]
     ):