diff --git a/sip.js b/sip.js
index 2bd73a0..9f279b9 100644
--- a/sip.js
+++ b/sip.js
@@ -1247,8 +1247,8 @@ function createClientTransaction(rq, transport, tu, cleanup) {
 
 function makeTransactionId(m) {
   if(m.method === 'ACK')
-    return ['INVITE', m.headers['call-id'], m.headers.via[0].params.branch].join();
-  return [m.headers.cseq.method, m.headers['call-id'], m.headers.via[0].params.branch].join();
+    return ['INVITE', m.headers['call-id'], m.headers.cseq.seq, m.headers.via[0].params.branch].join();
+  return [m.headers.cseq.method, m.headers['call-id'], m.headers.cseq.seq, m.headers.via[0].params.branch].join();
 }
 
 function makeTransactionLayer(options, transport) {