diff --git a/src/flow-monitor/model/ipv4-flow-probe.cc b/src/flow-monitor/model/ipv4-flow-probe.cc
index b9f4159ff..d12b8d502 100644
--- a/src/flow-monitor/model/ipv4-flow-probe.cc
+++ b/src/flow-monitor/model/ipv4-flow-probe.cc
@@ -327,7 +327,9 @@ Ipv4FlowProbe::QueueDropLogger (Ptr<const Packet> ipPayload)
   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;
   FlowId flowId = fTag.GetFlowId ();
   FlowPacketId packetId = fTag.GetPacketId ();
   uint32_t size = fTag.GetPacketSize ();