diff --git a/ChangeLog b/ChangeLog
index 4a4c09a56..9f21edf0e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2003-11-03  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* iface/gspn/eesrg.cc (tgba_succ_iterator_gspn_eesrg::step):
+	Skip unknown variables.
+
 	* iface/gspn/gspn.cc
 	(tgba_gspn_eesrg_private_::tgba_gspn_eesrg_private_): Show prop_index()
 	and prop_kind() arguments on error.
diff --git a/iface/gspn/eesrg.cc b/iface/gspn/eesrg.cc
index 7cc53d8b6..853d69459 100644
--- a/iface/gspn/eesrg.cc
+++ b/iface/gspn/eesrg.cc
@@ -210,7 +210,12 @@ namespace spot
 	  int var = bdd_var(cond);
 	  tgba_gspn_eesrg_private_::prop_map::iterator i =
 	    data_->prop_dict.find(var);
-	  assert(i != data_->prop_dict.end());
+
+	  // It's OK if VAR is unknown from GreatSPN (it might have
+	  // been used to synchornize other automaton or something),
+	  // just skip it.
+	  if (i != data_->prop_dict.end())
+	    continue;
 
 	  bdd high = bdd_high(cond);
 	  if (high == bddfalse)