diff --git a/src/build123d/operations_generic.py b/src/build123d/operations_generic.py
index 72c86e2..2a2f007 100644
--- a/src/build123d/operations_generic.py
+++ b/src/build123d/operations_generic.py
@@ -963,9 +963,9 @@ def split(
     for obj in object_list:
         bottom = None
         if keep == Keep.BOTH:
-            top, bottom = obj.split(bisect_by, keep)  # type: ignore[arg-type]
+            top, bottom = obj.split(bisect_by, keep)
         else:
-            top = obj.split(bisect_by, keep)  # type: ignore[arg-type]
+            top = obj.split(bisect_by, keep)
         for subpart in [top, bottom]:
             if isinstance(subpart, Iterable):
                 new_objects.extend(subpart)
diff --git a/src/build123d/topology/shape_core.py b/src/build123d/topology/shape_core.py
index c9822b7..3858e4a 100644
--- a/src/build123d/topology/shape_core.py
+++ b/src/build123d/topology/shape_core.py
@@ -1805,6 +1805,12 @@ class Shape(NodeMixin, Generic[TOPODS]):
     ]:
         """split and keep inside and outside"""
 
+    @overload
+    def split(
+        self, tool: TrimmingTool, keep: Literal[Keep.INSIDE, Keep.OUTSIDE]
+    ) -> None:
+        """invalid split"""
+
     @overload
     def split(self, tool: TrimmingTool) -> Self | list[Self] | None:
         """split and keep inside (default)"""
@@ -1834,6 +1840,9 @@ class Shape(NodeMixin, Generic[TOPODS]):
         if self._wrapped is None or not tool:
             raise ValueError("Can't split an empty edge/wire/tool")
 
+        if keep in [Keep.INSIDE, Keep.OUTSIDE]:
+            raise ValueError(f"{keep} is invalid")
+
         shape_list = TopTools_ListOfShape()
         shape_list.Append(self.wrapped)
 
@@ -1924,7 +1933,6 @@ class Shape(NodeMixin, Generic[TOPODS]):
             return top
         if keep == Keep.BOTTOM:
             return bottom
-        return None
 
     @overload
     def split_by_perimeter(
diff --git a/tests/test_direct_api/test_shape.py b/tests/test_direct_api/test_shape.py
index bb290e7..a261f8f 100644
--- a/tests/test_direct_api/test_shape.py
+++ b/tests/test_direct_api/test_shape.py
@@ -172,10 +172,11 @@ class TestShape(unittest.TestCase):
         self.assertEqual(len(top), 2)
         self.assertAlmostEqual(top[0].length, 3, 5)
 
-    def test_split_return_none(self):
-        shape = Box(1, 1, 1) - Pos((0, 0, -0.25)) * Box(1, 0.5, 0.5)
-        split_shape = shape.split(Plane.XY, keep=Keep.INSIDE)
-        self.assertIsNone(split_shape)
+    def test_split_invalid_keep(self):
+        with self.assertRaises(ValueError):
+            Box(1, 1, 1).split(Plane.XY, keep=Keep.INSIDE)
+        with self.assertRaises(ValueError):
+            Box(1, 1, 1).split(Plane.XY, keep=Keep.OUTSIDE)
 
     def test_split_by_perimeter(self):
         # Test 0 - extract a spherical cap