diff --git a/src/easyscience/variable/parameter.py b/src/easyscience/variable/parameter.py
index c459946c..2ff29877 100644
--- a/src/easyscience/variable/parameter.py
+++ b/src/easyscience/variable/parameter.py
@@ -314,8 +314,24 @@ def full_value(self) -> Variable:
         """
         if self._callback.fget is not None:
             scalar = self._callback.fget()
-            if scalar != self._scalar:
-                self._scalar = scalar
+            # Convert callback value to proper scipp Variable if needed
+            if not isinstance(scalar, Variable):
+                # If callback returns a raw number, convert it to a scipp scalar with the same unit as _scalar
+                scalar = sc.scalar(scalar, unit=self._scalar.unit)
+            elif str(scalar.unit) == 'dimensionless' and str(self._scalar.unit) != 'dimensionless':
+                # If callback returns dimensionless but original has unit, apply the original unit
+                scalar = sc.scalar(scalar.value, unit=self._scalar.unit, variance=scalar.variance)
+
+            try:
+                if scalar != self._scalar:
+                    self._scalar = scalar
+            except UnitError:
+                # If units are incompatible, update the scalar value only, preserving original unit
+                if hasattr(scalar, 'value'):
+                    self._scalar = sc.scalar(scalar.value, unit=self._scalar.unit,
+                                           variance=scalar.variance if hasattr(scalar, 'variance') else self._scalar.variance)
+                else:
+                    self._scalar = sc.scalar(scalar, unit=self._scalar.unit, variance=self._scalar.variance)
         return self._scalar
 
     @full_value.setter
diff --git a/tests/unit_tests/variable/test_parameter.py b/tests/unit_tests/variable/test_parameter.py
index 356ed83d..6687b108 100644
--- a/tests/unit_tests/variable/test_parameter.py
+++ b/tests/unit_tests/variable/test_parameter.py
@@ -656,6 +656,72 @@ def test_set_full_value(self, parameter: Parameter):
         with pytest.raises(AttributeError):
             parameter.full_value = sc.scalar(2, unit='s')
 
+    def test_full_value_callback_unit_corruption_fix(self):
+        """
+        Test that demonstrates the fix for the unit corruption issue when callbacks
+        return dimensionless values or raw numbers instead of proper scipp Variables.
+
+        This test covers the scenario where interface binding (generate_bindings)
+        sets up callbacks that return raw numeric values, which previously caused
+        UnitError when accessing Parameter.full_value.
+        """
+        # Given: A parameter with units (like thickness or roughness in EasyReflectometry)
+        parameter = Parameter(name="thickness", value=10.0, unit="Å", variance=0.01)
+
+        # Verify initial state
+        assert parameter.full_value == sc.scalar(10.0, unit='Å', variance=0.01)
+        assert parameter._scalar.unit == 'Å'
+
+        # Simulate the problematic callback scenario
+        mock_callback = MagicMock()
+
+        # Test Case 1: Callback returns raw numpy.float64 (the main issue)
+        mock_callback.fget.return_value = np.float64(15.0)  # dimensionless raw value
+        parameter._callback = mock_callback
+
+        # This should NOT raise UnitError anymore
+        full_value = parameter.full_value
+        assert full_value.value == 15.0
+        assert full_value.unit == 'Å'  # Unit should be preserved
+        assert parameter._scalar.unit == 'Å'  # Original unit maintained
+
+        # Test Case 2: Callback returns dimensionless scipp Variable
+        mock_callback.fget.return_value = sc.scalar(20.0)  # dimensionless scipp Variable
+
+        # This should also work without UnitError
+        full_value = parameter.full_value
+        assert full_value.value == 20.0
+        assert full_value.unit == 'Å'  # Unit should be applied from original
+        assert parameter._scalar.unit == 'Å'
+
+        # Test Case 3: Callback returns scipp Variable with correct unit
+        mock_callback.fget.return_value = sc.scalar(25.0, unit='Å')
+
+        # This should work as before
+        full_value = parameter.full_value
+        assert full_value.value == 25.0
+        assert full_value.unit == 'Å'
+        assert parameter._scalar.unit == 'Å'
+
+        # Test Case 4: Callback returns incompatible unit (fallback behavior)
+        mock_callback.fget.return_value = sc.scalar(30.0, unit='s')  # incompatible unit
+
+        # Should handle gracefully by preserving original unit and updating value
+        full_value = parameter.full_value
+        assert full_value.value == 30.0
+        assert full_value.unit == 'Å'  # Original unit preserved
+
+        # Test Case 5: Verify arithmetic operations work after callback scenario
+        parameter2 = Parameter(name="roughness", value=3.0, unit="Å", variance=0.01)
+        mock_callback2 = MagicMock()
+        mock_callback2.fget.return_value = np.float64(5.0)  # raw dimensionless
+        parameter2._callback = mock_callback2
+
+        # Both parameters should work in arithmetic operations
+        result = parameter.full_value + parameter2.full_value
+        assert result.unit == 'Å'
+        assert result.value == 35.0  # 30.0 + 5.0
+
     def test_set_variance_dependent_parameter(self, normal_parameter: Parameter):
         # When
         dependent_parameter = Parameter.from_dependency(