diff --git a/dune/solvers/iterationsteps/mmgstep.cc b/dune/solvers/iterationsteps/mmgstep.cc
index b8622b43e2caebdc857aae36f8715d39cb7b3af6..982b6b8fdcd0484151ac4f219c382d2dba30c547 100644
--- a/dune/solvers/iterationsteps/mmgstep.cc
+++ b/dune/solvers/iterationsteps/mmgstep.cc
@@ -211,7 +211,7 @@ void MonotoneMGStep<OperatorType, DiscFuncType>::iterate()
         
 #ifndef NDEBUG
         // Debug: is the current iterate really admissible?
-        for (int i=0; i<obstacles[level].size(); i++)
+        for (size_t i=0; i<obstacles[level].size(); i++)
             for (int j=0; j<DiscFuncType::block_type::dimension; j++)
                 if (x[level][i][j] < obstacles[level][i].lower(j)
                     || x[level][i][j] > obstacles[level][i].upper(j)) {