diff --git a/torch/distributed/device_mesh.py b/torch/distributed/device_mesh.py
index c90dba2220c5..9fef00f5a809 100644
--- a/torch/distributed/device_mesh.py
+++ b/torch/distributed/device_mesh.py
@@ -239,7 +239,9 @@ else:
                 )
             return not_none(device_mesh.mesh_dim_names.index(mesh_dim_name))
 
-        def _get_slice_mesh_layout(self, device_mesh, mesh_dim_names) -> _MeshLayout:
+        def _get_slice_mesh_layout(
+            self, device_mesh: "DeviceMesh", mesh_dim_names: tuple[str, ...]
+        ) -> _MeshLayout:
             """
             Validate whether the mesh_dim_names is valid for slicing the given device_mesh.
             If valid, return dim indexes of the slice mesh in the device mesh.
@@ -266,7 +268,7 @@ else:
                 else {}
             )
             valid_mesh_dim_names = [
-                *device_mesh.mesh_dim_names,
+                *not_none(device_mesh.mesh_dim_names),
                 *flatten_name_to_root_layout,
             ]
 
@@ -281,11 +283,17 @@ else:
 
             layout_sliced = []
             for name in mesh_dim_names:
-                if name in device_mesh.mesh_dim_names:
+                if name in not_none(device_mesh.mesh_dim_names):
                     layout_sliced.append(
-                        device_mesh._layout[device_mesh.mesh_dim_names.index(name)]
+                        device_mesh._layout[
+                            not_none(device_mesh.mesh_dim_names).index(name)
+                        ]
                     )
                 elif name in flatten_name_to_root_layout:
+                    warnings.warn(
+                        "Slicing a flattened dim from root mesh will be deprecated in PT 2.11. "
+                        "Users need to bookkeep the flattened mesh directly. "
+                    )
                     layout_sliced.append(flatten_name_to_root_layout[name])
 
             sliced_sizes = tuple(l.sizes for l in layout_sliced)