diff --git a/conversions/octal_to_binary.py b/conversions/octal_to_binary.py
index 48075b8df..e7e841f12 100644
--- a/conversions/octal_to_binary.py
+++ b/conversions/octal_to_binary.py
@@ -1,4 +1,4 @@
-def octal_to_binary(octal_number):
+def octal_to_binary(octal_number: str) -> str:
     """
     Convert an octal number to binary.