diff --git a/src/tgbaalgos/minimize.cc b/src/tgbaalgos/minimize.cc
index 35e97d9e5..0cd19a9da 100644
--- a/src/tgbaalgos/minimize.cc
+++ b/src/tgbaalgos/minimize.cc
@@ -619,7 +619,7 @@ namespace spot
     return minimize_dfa(det_a, final, non_final);
   }
 
-  const tgba*
+  tgba*
   minimize_obligation(const tgba* aut_f,
 		      const ltl::formula* f, const tgba* aut_neg_f)
   {
@@ -706,6 +706,6 @@ namespace spot
     if (ok)
       return min_aut_f;
     delete min_aut_f;
-    return aut_f;
+    return const_cast<tgba*>(aut_f);
   }
 }
diff --git a/src/tgbaalgos/minimize.hh b/src/tgbaalgos/minimize.hh
index 8ac29091a..e2d43f4c9 100644
--- a/src/tgbaalgos/minimize.hh
+++ b/src/tgbaalgos/minimize.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2009, 2010, 2011 Laboratoire de Recherche et Développement
+// Copyright (C) 2009, 2010, 2011, 2012 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -140,9 +140,9 @@ namespace spot
   /// product(aut_neg_f,minize(aut))</code> are both empty.  If they
   /// are, the the minimization was sound.  (See the paper for full
   /// details.)
-  const tgba* minimize_obligation(const tgba* aut_f,
-				  const ltl::formula* f = 0,
-				  const tgba* aut_neg_f = 0);
+  tgba* minimize_obligation(const tgba* aut_f,
+			    const ltl::formula* f = 0,
+			    const tgba* aut_neg_f = 0);
 
   /// @}
 }