--- a/src/wifi/model/mac-low.cc	Wed May 13 12:02:14 2015 -0700
+++ b/src/wifi/model/mac-low.cc	Sun May 17 12:50:59 2015 +0200
@@ -2847,8 +2847,11 @@
 void
 MacLow::FlushAggregateQueue (void)
 {
-  NS_LOG_DEBUG("Flush aggregate queue");
-  m_aggregateQueue->Flush ();
+  if (m_aggregateQueue->GetSize () > 0)
+  {
+    NS_LOG_DEBUG("Flush aggregate queue");
+    m_aggregateQueue->Flush ();
+  }
   m_txPackets.clear ();
 }