diff --git a/include/gsup_protocol.hrl b/include/gsup_protocol.hrl
index dc49723..2e9c9d3 100644
--- a/include/gsup_protocol.hrl
+++ b/include/gsup_protocol.hrl
@@ -219,7 +219,7 @@
   16#44 => #{message_type => e_forward_access_signalling_req, mandatory => [message_class, source_name, destination_name, an_apdu, session_id, session_state]},
   16#47 => #{message_type => e_close, mandatory => [message_class, source_name, destination_name, session_id, session_state]},
   16#4b => #{message_type => e_abort, mandatory => [message_class, session_id, session_state, bssap_cause]},
-  16#4e => #{message_type => e_routing_err, mandatory => [message_class, source_name, destination_name, session_id, session_state]}
+  16#4e => #{message_type => e_routing_err, mandatory => [message_class, source_name, destination_name], optional => [session_id, session_state]}
 }).
 
 -define(AUTH_TUPLE_MANDATORY, [rand, sres, kc]).