diff --git a/dune/istl/paamg/aggregates.hh b/dune/istl/paamg/aggregates.hh
index 769a76023302dd692add7ea16b37a52a46743777..e5005386f6e59aea6e435a4f1d19dbf2a9e344bd 100644
--- a/dune/istl/paamg/aggregates.hh
+++ b/dune/istl/paamg/aggregates.hh
@@ -1504,8 +1504,9 @@ namespace Dune
     template<class G,class S>
     inline void Aggregate<G,S>::add(std::vector<Vertex>& vertices)
     {
+#ifndef NDEBUG
       std::size_t oldsize = vertices_.size();
-
+#endif
       typedef typename std::vector<Vertex>::iterator Iterator;
 
       typedef typename VertexSet::iterator SIterator;