diff --git a/dune/solvers/operators/sumoperator.hh b/dune/solvers/operators/sumoperator.hh
index e6e978b7076131c905f1067d08e727722425bf20..4e909c7088ab9224c46727cdacdb7eba3f971fd8 100644
--- a/dune/solvers/operators/sumoperator.hh
+++ b/dune/solvers/operators/sumoperator.hh
@@ -67,13 +67,13 @@ class SumOperator
         //! return the number of rows
         const size_t N() const
         {
-            sparse_matrix_->N();
+            return sparse_matrix_->N();
         }
 
         //! return the number of columns
         const size_t M() const
         {
-            sparse_matrix_->M();
+            return sparse_matrix_->M();
         }
 
         //! return one summand