diff --git a/sorts/unknown_sort.py b/sorts/unknown_sort.py
index 9fa9d22fb5e0..3545da68ea80 100644
--- a/sorts/unknown_sort.py
+++ b/sorts/unknown_sort.py
@@ -6,7 +6,7 @@
 """
 
 
-def merge_sort(collection):
+def merge_sort(collection: list) -> list:
     """Pure implementation of the fastest merge sort algorithm in Python
 
     :param collection: some mutable ordered collection with heterogeneous