diff --git a/sphinx/testing/path.py b/sphinx/testing/path.py
index 3a9ee8ddb..49f0ffa60 100644
--- a/sphinx/testing/path.py
+++ b/sphinx/testing/path.py
@@ -178,7 +178,7 @@ class path(str):
         with open(self, mode='rb') as f:
             return f.read()
 
-    def write_bytes(self, bytes: str, append: bool = False) -> None:
+    def write_bytes(self, bytes: bytes, append: bool = False) -> None:
         """
         Writes the given `bytes` to the file.