diff --git a/dune/solvers/iterationsteps/mmgstep.hh b/dune/solvers/iterationsteps/mmgstep.hh
index b7917a8f295ff8f6b24135b34803d3f73df793a5..3cbd0cb6106bb0846d810ce059c9d1bd835f7e84 100644
--- a/dune/solvers/iterationsteps/mmgstep.hh
+++ b/dune/solvers/iterationsteps/mmgstep.hh
@@ -20,12 +20,14 @@ class MonotoneMGStep : public MultigridStep<MatrixType, VectorType>
     typedef std::vector<BoxConstraint<field_type,dim> > ObstacleVectorType;
 public:
     
-    MonotoneMGStep() {}
+    MonotoneMGStep() : hasObstacleHierarchy_(0), obstacleHierarchy_(0)
+    {}
     
     MonotoneMGStep(const MatrixType& mat,
                    VectorType& x,
                    VectorType& rhs)
-        : MultigridStep<MatrixType, VectorType>(mat, x, rhs)
+        : MultigridStep<MatrixType, VectorType>(mat, x, rhs),
+          hasObstacleHierarchy_(0), obstacleHierarchy_(0)
     {
         oldCritical.resize(x.size(), false);
     }