diff --git a/pydantic_extra_types/isbn.py b/pydantic_extra_types/isbn.py
index 81b3648b..df573c68 100644
--- a/pydantic_extra_types/isbn.py
+++ b/pydantic_extra_types/isbn.py
@@ -123,7 +123,7 @@ def validate_isbn_format(value: str) -> None:
             if isbn10_digit_calc(value) != value[-1]:
                 raise PydanticCustomError('isbn_invalid_digit_check_isbn10', 'Provided digit is invalid for given ISBN')
 
-        elif isbn_length == 13:
+        if isbn_length == 13:
             if not value.isdigit():
                 raise PydanticCustomError('isbn13_invalid_characters', 'All digits of ISBN-13 must be integers')
             if value[:3] not in ('978', '979'):