Consider  a server replying to a method call with an error message generated
like this:

  reply = dbus_message_new_error(i->message,
"org.freedesktop.Avahi.TimeoutError", NULL);

(i.e. error_message is NULL (!))

If a python-dbus based client issued this method call asynchronously, the
specified error function ist not called. Instead "Exception
exceptions.IndexError: 'list index out of range' in
'dbus_bindings._pending_call_notification' ignored
" is printed. If the error_message on the server side is non-NULL the error is
delivered correctly. If the method call is executed synchronously the error is
delivered correctly, regardless of error_message.