diff --git a/dune/solvers/iterationsteps/multigridstep.hh b/dune/solvers/iterationsteps/multigridstep.hh
index 36231844a5de53c01b502da1f1188b6aa20d579c..3b6c8042a22726ee44776892ba3fb423bb65ef2f 100644
--- a/dune/solvers/iterationsteps/multigridstep.hh
+++ b/dune/solvers/iterationsteps/multigridstep.hh
@@ -112,7 +112,7 @@
 
             for (int i=0; i<int(mat_.size())-1; i++)
             {
-                mat_[i] = NULL;
+                mat_[i].reset();
                 ignoreNodesHierarchy_[i] = NULL;
             }