--- a/rdkit/Chem/__init__.pyi
+++ b/rdkit/Chem/__init__.pyi
@@ -138,7 +138,7 @@
         ...
     def _sizeCalc(self):
         ...
-def CanonSmiles(smi, useChiral = 1):
+def CanonSmiles(smi: str, useChiral: int = 1) -> str:
     """
     A convenience function for canonicalizing SMILES
