--- a/src/flow-monitor/model/ipv4-flow-probe.cc	Sun Feb 05 09:23:51 2012 -0800
+++ b/src/flow-monitor/model/ipv4-flow-probe.cc	Wed Feb 08 11:12:17 2012 +0100
@@ -330,10 +330,11 @@
   // ConstCast: see http://www.nsnam.org/bugzilla/show_bug.cgi?id=904
   bool tagFound;
   tagFound = ConstCast<Packet> (ipPayload)->RemovePacketTag (fTag);
-  NS_ASSERT_MSG (tagFound, "FlowProbeTag is missing");
-  // cast tagFound to void, to suppress 'tagFound' set but not used compiler 
-  // warning in optimized builds
-  (void) tagFound;
+  if (!tagFound)
+    {
+      return;
+    }
+
   FlowId flowId = fTag.GetFlowId ();
   FlowPacketId packetId = fTag.GetPacketId ();
   uint32_t size = fTag.GetPacketSize ();