diff --git a/Doc/library/select.rst b/Doc/library/select.rst
index 48e289a..6dbf16f 100644
--- a/Doc/library/select.rst
+++ b/Doc/library/select.rst
@@ -154,6 +154,11 @@
 
    Register a fd descriptor with the epoll object.
 
+   .. note::
+
+     Registering a file descriptor that's already registered raises an
+     IOError -- contrary to :ref:`poll-objects`'s register.
+
 
 .. method:: epoll.modify(fd, eventmask)