diff --git a/Doc/library/imaplib.rst b/Doc/library/imaplib.rst
index 3f45c95..da6cc8c 100644
--- a/Doc/library/imaplib.rst
+++ b/Doc/library/imaplib.rst
@@ -176,9 +176,10 @@
 
       data = authobject(response)
 
-   It will be called to process server continuation responses. It should return
-   ``data`` that will be encoded and sent to server. It should return ``None`` if
-   the client abort response ``*`` should be sent instead.
+   It will be called to process server continuation responses; the *response*
+   argument it is passed will be ``bytes``.  It should return ``bytes`` *data*
+   that will be base64 encoded and sent to the server.  It should return
+   ``None`` if the client abort response ``*`` should be sent instead.
 
 
 .. method:: IMAP4.check()