It's possible that the Misc plugin isn't loaded.

src/callbacks.py

@@ -841,6 +841,7 @@ class Privmsg(irclib.IrcCallback):
                     self._original(irc, msg, args)
                 else:
                     cb = irc.getCallback('Misc')
+                    if cb is not None:
                         cb.help(irc, msg, [self.name()])
             if args:
                 name = canonicalName(args[0])