diff --git a/ChangeLog b/ChangeLog
index 4766cfa4b..5efcf9815 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2006-02-10  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* src/tgbaparse/tgbaparse.yy (acc_decl): Diagnose acceptance
+	conditions rejected by the environment.
+
 	* iface/gspn/ltlgspn.cc (display_stats): New function.
 	(main): Use it.
 	* iface/gspn/ssp.cc: Add more counters for statistics.
diff --git a/src/tgbaparse/tgbaparse.yy b/src/tgbaparse/tgbaparse.yy
index af1702ed3..1eea064fd 100644
--- a/src/tgbaparse/tgbaparse.yy
+++ b/src/tgbaparse/tgbaparse.yy
@@ -1,6 +1,6 @@
-/* Copyright (C) 2003, 2004, 2005  Laboratoire d'Informatique de Paris 6 (LIP6),
-** département Systèmes Répartis Coopératifs (SRC), Université Pierre
-** et Marie Curie.
+/* Copyright (C) 2003, 2004, 2005, 2006 Laboratoire d'Informatique de
+** Paris 6 (LIP6), département Systèmes Répartis Coopératifs (SRC),
+** Université Pierre et Marie Curie.
 **
 ** This file is part of Spot, a model checking library.
 **
@@ -196,6 +196,16 @@ acc_decl:
        | acc_decl strident
        {
 	 formula* f = parse_environment.require(*$2);
+	 if (! f)
+	   {
+	     std::string s = "acceptance condition `";
+	     s += *$2;
+	     s += "' unknown in environment `";
+	     s += parse_environment.name();
+	     s += "'";
+	     error_list.push_back(spot::tgba_parse_error(@2, s));
+	     YYERROR;
+	   }
 	 result->declare_acceptance_condition(f);
 	 delete $2;
        }