diff --git a/docs/compat.rst b/docs/compat.rst
index 54c3cb2..5e921f5 100644
--- a/docs/compat.rst
+++ b/docs/compat.rst
@@ -56,7 +56,7 @@ Compat
     Same to ``itertools.imap`` on PY2 or ``map`` on PY3.
 
 
-.. py:function:: pydu.compat.cmp
+.. py:function:: pydu.compat.cmp(x, y)
 
     Same to ``cmp`` on PY2, but implement on PY3.