diff --git a/readme_renderer/clean.py b/readme_renderer/clean.py
index a123044..3335ad4 100644
--- a/readme_renderer/clean.py
+++ b/readme_renderer/clean.py
@@ -83,7 +83,7 @@ def clean(
             link_rel="nofollow",
             url_schemes={"http", "https", "mailto"},
         )
-
-        return cleaned
     except ValueError:
         return None
+    else:
+        return cleaned
diff --git a/readme_renderer/rst.py b/readme_renderer/rst.py
index 52621a3..488357b 100644
--- a/readme_renderer/rst.py
+++ b/readme_renderer/rst.py
@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import io
-from typing import Any, Dict, IO, Optional
+from typing import Any, ClassVar, Dict, IO, Optional
 
 from docutils.core import publish_parts
 from docutils.nodes import Element
@@ -26,7 +26,7 @@
 class ReadMeHTMLTranslator(HTMLTranslator):
 
     # Overrides base class not to output `<object>` tag for SVG images.
-    object_image_types: Dict[str, str] = {}
+    object_image_types: ClassVar[Dict[str, str]] = {}
 
     def emptytag(
         self,