--- a/src/common/packet-metadata.cc	Thu Jun 19 14:58:46 2008 -0700
+++ b/src/common/packet-metadata.cc	Fri Jun 20 15:06:23 2008 -0700
@@ -726,6 +726,12 @@
       *this = o;
       return;
     }
+  if (o.m_head == 0xffff)
+    {
+      NS_ASSERT (o.m_tail == 0xffff);
+      // we have nothing to append.
+      return;
+    }
   NS_ASSERT (m_head != 0xffff && m_tail != 0xffff);
 
   // We read the current tail because we are going to append