diff --git a/Lib/bz2.py b/Lib/bz2.py
index 5c228f9..7e1a7e2 100644
--- a/Lib/bz2.py
+++ b/Lib/bz2.py
@@ -40,7 +40,7 @@
     """
 
     def __init__(self, filename=None, mode="r", buffering=None,
-                 compresslevel=9, fileobj=None):
+                 compresslevel=9, *, fileobj=None):
         """Open a bzip2-compressed file.
 
         If filename is given, open the named file. Otherwise, operate on