diff --git a/src/build123d/build_common.py b/src/build123d/build_common.py
index c5c4f87f..0c7dfbd4 100644
--- a/src/build123d/build_common.py
+++ b/src/build123d/build_common.py
@@ -49,7 +49,7 @@
 from abc import ABC, abstractmethod
 from itertools import product
 from math import sqrt
-from typing import Callable, Iterable, Optional, Union, TypeVar
+from typing import Any, Callable, Iterable, Optional, Union, TypeVar
 from typing_extensions import Self, ParamSpec, Concatenate
 
 from build123d.build_enums import Align, Mode, Select
@@ -103,24 +103,26 @@
 KG = 1000 * G
 LB = 453.59237 * G
 
-T = TypeVar("T")
 
+def _is_point(obj):
+    """Identify points as tuples of numbers"""
+    return isinstance(obj, tuple) and all(
+        isinstance(item, (int, float)) for item in obj
+    )
 
-def flatten_sequence(*obj: T) -> list[T]:
-    """Convert a sequence of object potentially containing iterables into a flat list"""
 
-    def is_point(obj):
-        """Identify points as tuples of numbers"""
-        return isinstance(obj, tuple) and all(
-            isinstance(item, (int, float)) for item in obj
-        )
+T = TypeVar("T", Any, list[Any])
+
+
+def flatten_sequence(*obj: T) -> list[Any]:
+    """Convert a sequence of object potentially containing iterables into a flat list"""
 
     flat_list = []
     for item in obj:
         # Note: an Iterable can't be used here as it will match with Vector & Vertex
         # and break them into a list of floats.
-        if isinstance(item, (list, tuple, filter, set)) and not is_point(item):
-            flat_list.extend(item)
+        if isinstance(item, (list, tuple, filter, set)) and not _is_point(item):
+            flat_list.extend(flatten_sequence(*item))
         else:
             flat_list.append(item)
 
diff --git a/tests/test_build_common.py b/tests/test_build_common.py
index 668fef1c..ef3eb331 100644
--- a/tests/test_build_common.py
+++ b/tests/test_build_common.py
@@ -71,6 +71,24 @@ def test_points(self):
             flatten_sequence("a", (1.0, 2.0, 3.0), "e"), ["a", (1.0, 2.0, 3.0), "e"]
         )
 
+    def test_group_slice(self):
+        with BuildSketch() as s3:
+            Circle(55 / 2 + 3)
+            Rectangle(23 + 6, 42, align=(Align.CENTER, Align.MIN))
+            Circle(55 / 2, mode=Mode.SUBTRACT)
+            Rectangle(23, 42, mode=Mode.SUBTRACT, align=(Align.CENTER, Align.MIN))
+            vertex_groups = s3.vertices().group_by(Axis.Y)[0:2]
+
+        self.assertListEqual(
+            flatten_sequence(vertex_groups),
+            [
+                vertex_groups[0][0],
+                vertex_groups[0][1],
+                vertex_groups[1][0],
+                vertex_groups[1][1],
+            ],
+        )
+
 
 class TestBuilder(unittest.TestCase):
     """Test the Builder base class"""