diff --git a/protein_design_tools/utils/helpers.py b/protein_design_tools/utils/helpers.py
index 2835463..553ef29 100644
--- a/protein_design_tools/utils/helpers.py
+++ b/protein_design_tools/utils/helpers.py
@@ -4,7 +4,7 @@
 
 
 def parse_residue_selection(
-    selection: Union[Dict[str, List[Union[int, range]]], None]
+    selection: Union[Dict[str, List[Union[int, range]]], None],
 ) -> Dict[str, List[int]]:
     """
     Parse residue selection input into a standardized dictionary.