diff --git a/src/network/transportfragment.cc b/src/network/transportfragment.cc
index 55cd41d..58f2975 100644
--- a/src/network/transportfragment.cc
+++ b/src/network/transportfragment.cc
@@ -136,7 +136,7 @@ bool Fragment::operator==( const Fragment &x )
     && ( initialized == x.initialized ) && ( contents == x.contents );
 }
 
-vector<Fragment> Fragmenter::make_fragments( Instruction &inst, int MTU )
+vector<Fragment> Fragmenter::make_fragments( const Instruction &inst, int MTU )
 {
   if ( (inst.old_num() != last_instruction.old_num())
        || (inst.new_num() != last_instruction.new_num())
diff --git a/src/network/transportfragment.h b/src/network/transportfragment.h
index e3b2739..f55ee67 100644
--- a/src/network/transportfragment.h
+++ b/src/network/transportfragment.h
@@ -88,7 +88,7 @@ namespace Network {
       last_instruction.set_old_num( -1 );
       last_instruction.set_new_num( -1 );
     }
-    vector<Fragment> make_fragments( Instruction &inst, int MTU );
+    vector<Fragment> make_fragments( const Instruction &inst, int MTU );
     uint64_t last_ack_sent( void ) const { return last_instruction.ack_num(); }
   };