diff --git a/src/rod/sdf/geometry.py b/src/rod/sdf/geometry.py
index 767b2a7..de55d58 100644
--- a/src/rod/sdf/geometry.py
+++ b/src/rod/sdf/geometry.py
@@ -1,7 +1,8 @@
 from __future__ import annotations
 
 import dataclasses
-from typing import ClassVar
+import types
+from typing import ClassVar, Union
 
 import mashumaro
 
@@ -116,7 +117,7 @@ class Sphere(Element):
 @dataclasses.dataclass
 class Geometry(Element):
 
-    GeometryType: ClassVar = (
+    GeometryType: ClassVar[types.UnionType] = (
         Box | Capsule | Cylinder | Ellipsoid | Heightmap | Mesh | Plane | Sphere
     )
 
diff --git a/src/rod/tree/tree_elements.py b/src/rod/tree/tree_elements.py
index 400ea8a..f09958a 100644
--- a/src/rod/tree/tree_elements.py
+++ b/src/rod/tree/tree_elements.py
@@ -57,7 +57,7 @@ def __str__(self) -> str:
         content_string = (
             f"name={self.name()}, "
             f"index={self.index}, "
-            f"parent={self.parent.name() if self.parent else "None"}, "
+            f"parent={self.parent.name() if self.parent else str(None)}, "
             f"children=[{[c.name() for c in self.children]}]"
         )
 
diff --git a/src/rod/urdf/exporter.py b/src/rod/urdf/exporter.py
index a1ba3ed..06ecdc8 100644
--- a/src/rod/urdf/exporter.py
+++ b/src/rod/urdf/exporter.py
@@ -446,7 +446,7 @@ def _rod_material_to_xmltodict(material: rod.Material) -> dict[str, Any]:
             return UrdfExporter.DefaultMaterial
 
         return {
-            "@name": f"color_{hash(" ".join(map(str, material.diffuse)))}",
+            "@name": f"color_{hash(' '.join(map(str, material.diffuse)))}",
             "color": {
                 "@rgba": " ".join(map(str, material.diffuse)),
             },