diff --git a/src/ltlvisit/apcollect.cc b/src/ltlvisit/apcollect.cc
index 87cb44c74..16c8e4479 100644
--- a/src/ltlvisit/apcollect.cc
+++ b/src/ltlvisit/apcollect.cc
@@ -72,7 +72,7 @@ namespace spot
     }
 
     bdd
-    atomic_prop_collect_as_bdd(const formula* f, const_tgba_ptr a)
+    atomic_prop_collect_as_bdd(const formula* f, const const_tgba_ptr& a)
     {
       spot::ltl::atomic_prop_set aps;
       atomic_prop_collect(f, &aps);
diff --git a/src/ltlvisit/apcollect.hh b/src/ltlvisit/apcollect.hh
index 6fac7d6cd..8e591f484 100644
--- a/src/ltlvisit/apcollect.hh
+++ b/src/ltlvisit/apcollect.hh
@@ -64,7 +64,8 @@ namespace spot
     /// \param a that automaton that should register the BDD variables used.
     /// \return A conjunction the atomic propositions.
     SPOT_API bdd
-    atomic_prop_collect_as_bdd(const formula* f, const_tgba_ptr a);
+    atomic_prop_collect_as_bdd(const formula* f,
+			       const const_tgba_ptr& a);
 
     /// @}
   }