diff --git a/src/corva_unit_converter/converter.py b/src/corva_unit_converter/converter.py
index 62c96f4..af4cef3 100644
--- a/src/corva_unit_converter/converter.py
+++ b/src/corva_unit_converter/converter.py
@@ -62,7 +62,7 @@ def get_unit_definition(self, source, unit, measure):
         return definition
 
     def convert(self, unit_from: str, unit_to: str,  # noqa: CCR001, CFQ004
-                value: Union[int, float], measure: str = None):
+                value: Union[int, float], measure: Union[str, None] = None):
         """Main function"""
         if measure is not None and measure not in measures:
             invalid_measure_msg = (
diff --git a/tests/test_converter.py b/tests/test_converter.py
index 8d6901f..1da0b42 100644
--- a/tests/test_converter.py
+++ b/tests/test_converter.py
@@ -1,4 +1,5 @@
 from unittest.mock import patch
+
 from src.corva_unit_converter.converter import Converter