diff --git a/core/AmSession.cpp b/core/AmSession.cpp
index 7963453c..cb92abf6 100644
--- a/core/AmSession.cpp
+++ b/core/AmSession.cpp
@@ -1216,16 +1216,23 @@ void AmSession::onFailure(AmSipDialogEventHandler::FailureCause cause,
     const AmSipRequest *req, const AmSipReply *rpl)
 {
   switch (cause) {
-    case FAIL_REL100:
+    case FAIL_REL100_421:
+    case FAIL_REL100_420:
       if (rpl) {
         dlg.cancel();
         if (dlg.getStatus() < AmSipDialog::Connected)
           setStopped();
       } else if (req) {
-          dlg.reply(*req, 421, "Extension Required", "", "",
+        if (cause == FAIL_REL100_421) {
+          dlg.reply(*req, 421, SIP_REPLY_EXTENSION_REQUIRED, "", "",
               SIP_HDR_COLSP(SIP_HDR_REQUIRE) SIP_EXT_100REL CRLF);
-          if (dlg.getStatus() < AmSipDialog::Connected)
-            setStopped();
+        } else {
+          dlg.reply(*req, 420, SIP_REPLY_BAD_EXTENSION, "", "",
+              SIP_HDR_COLSP(SIP_HDR_UNSUPPORTED) SIP_EXT_100REL CRLF);
+        }
+        /* finally, stop session if running */
+        if (dlg.getStatus() < AmSipDialog::Connected)
+          setStopped();
       }
       break;
     default:
diff --git a/core/AmSipDialog.cpp b/core/AmSipDialog.cpp
index 3a79d3ac..fa73dc1a 100644
--- a/core/AmSipDialog.cpp
+++ b/core/AmSipDialog.cpp
@@ -173,19 +173,17 @@ int AmSipDialog::rel100OnRequestIn(const AmSipRequest& req)
               SIP_EXT_100REL))) {
           ERROR("'" SIP_EXT_100REL "' extension required, but not advertised"
             " by peer.\n");
-          if (hdl) hdl->onFailure(FAIL_REL100, &req, 0);
+          if (hdl) hdl->onFailure(FAIL_REL100_421, &req, 0);
           return 0; // has been replied
         }
         break; // 100rel required
 
       case REL100_DISABLED:
         // TODO: shouldn't this be part of a more general check in SEMS?
-        if (key_in_list(getHeader(req.hdrs,SIP_HDR_REQUIRE),SIP_EXT_100REL))
-          reply_error(req, 420, SIP_REPLY_BAD_EXTENSION, 
-		      SIP_HDR_COLSP(SIP_HDR_UNSUPPORTED) SIP_EXT_100REL CRLF,
-		      next_hop_for_replies ? next_hop_ip : "",
-		      next_hop_for_replies ? next_hop_port : 0);
-	return 0;
+        if (key_in_list(getHeader(req.hdrs,SIP_HDR_REQUIRE),SIP_EXT_100REL)) {
+          if (hdl) hdl->onFailure(FAIL_REL100_420, &req, 0);
+          return 0; // has been replied
+        }
         break;
 
       default:
@@ -416,7 +414,7 @@ int AmSipDialog::rel100OnReplyIn(const AmSipReply &reply)
           !reply.rseq) {
         ERROR(SIP_EXT_100REL " not supported or no positive RSeq value in "
             "(reliable) 1xx.\n");
-        if (hdl) hdl->onFailure(FAIL_REL100, 0, &reply);
+        if (hdl) hdl->onFailure(FAIL_REL100_421, 0, &reply);
       } else {
         DBG(SIP_EXT_100REL " now active.\n");
         if (hdl) hdl->onInvite1xxRel(reply);
@@ -436,7 +434,7 @@ int AmSipDialog::rel100OnReplyIn(const AmSipReply &reply)
   } else if (reliable_1xx && reply.method==SIP_METH_PRACK) {
     if (300 <= reply.code) {
       // if PRACK fails, tear down session
-      if (hdl) hdl->onFailure(FAIL_REL100, 0, &reply);
+      if (hdl) hdl->onFailure(FAIL_REL100_421, 0, &reply);
     } else if (200 <= reply.code) {
       if (hdl) hdl->onPrack2xx(reply);
     } else {
diff --git a/core/AmSipDialog.h b/core/AmSipDialog.h
index 52f2a362..baceb2ca 100644
--- a/core/AmSipDialog.h
+++ b/core/AmSipDialog.h
@@ -110,8 +110,10 @@ class AmSipDialogEventHandler
   virtual void onPrack2xx(const AmSipReply &)=0;
 
   enum FailureCause {
-    FAIL_REL100,
-#define FAIL_REL100  AmSipDialogEventHandler::FAIL_REL100
+    FAIL_REL100_421,
+#define FAIL_REL100_421  AmSipDialogEventHandler::FAIL_REL100_421
+    FAIL_REL100_420,
+#define FAIL_REL100_420  AmSipDialogEventHandler::FAIL_REL100_420
   };
   virtual void onFailure(FailureCause cause, const AmSipRequest*, 
       const AmSipReply*)=0;
diff --git a/core/AmSipHeaders.h b/core/AmSipHeaders.h
index 28ac91a0..cb8543cc 100644
--- a/core/AmSipHeaders.h
+++ b/core/AmSipHeaders.h
@@ -52,4 +52,5 @@
 
 #define SIP_REPLY_SERVER_INTERNAL_ERROR "Server Internal Error"
 #define SIP_REPLY_BAD_EXTENSION         "Bad Extension"
+#define SIP_REPLY_EXTENSION_REQUIRED    "Extension Required"
 #endif /* __AMSIPHEADERS_H__ */