diff --git a/transportfragment.hpp b/transportfragment.hpp
index 8304360..7d18008 100644
--- a/transportfragment.hpp
+++ b/transportfragment.hpp
@@ -64,7 +64,11 @@ namespace Network {
     int last_MTU;
 
   public:
-    Fragmenter() : next_instruction_id( 0 ), last_instruction(), last_MTU( -1 ) {}
+    Fragmenter() : next_instruction_id( 0 ), last_instruction(), last_MTU( -1 )
+    {
+      last_instruction.set_old_num( -1 );
+      last_instruction.set_new_num( -1 );
+    }
     vector<Fragment> make_fragments( Instruction &inst, int MTU );
     uint64_t last_ack_sent( void ) { return last_instruction.ack_num(); }
   };