diff --git a/vcg/simplex/faceplus/base.h b/vcg/simplex/faceplus/base.h
index 101db319..dccad32d 100644
--- a/vcg/simplex/faceplus/base.h
+++ b/vcg/simplex/faceplus/base.h
@@ -109,8 +109,8 @@ class FaceTypeHolder{
 
  // prot
  	const int VN()  const { return 3;}
-	inline int Prev(const int & i){ return (i+(3-1))%3;}
-	inline int Next(const int & i){ return (i+1)%3;}
+	inline const int Prev(const int & i) const { return (i+(3-1))%3;}
+	inline const int Next(const int & i) const { return (i+1)%3;}
 	inline void Alloc(const int & ){}
 	inline void Dealloc(){}
 };