diff --git a/Doc/lib/libxmlrpclib.tex b/Doc/lib/libxmlrpclib.tex
index d8126e4..0c25c02 100644
--- a/Doc/lib/libxmlrpclib.tex
+++ b/Doc/lib/libxmlrpclib.tex
@@ -58,7 +58,9 @@
 This is the full set of data types supported by XML-RPC.  Method calls
 may also raise a special \exception{Fault} instance, used to signal
 XML-RPC server errors, or \exception{ProtocolError} used to signal an
-error in the HTTP/HTTPS transport layer.
+error in the HTTP/HTTPS transport layer.  Note that even though starting
+with Python 2.2 you can subclass builtin types, the xmlrpclib module
+currently does not marshal instances of such subclasses.
 
 When passing strings, characters special to XML such as \samp{<},
 \samp{>}, and \samp{\&} will be automatically escaped.  However, it's