diff --git a/dune/solvers/norms/twonorm.hh b/dune/solvers/norms/twonorm.hh
index bb605c7e87c57f4dec54b495c6d074856cc4d67b..65b41dadcb06a4fc036295c4f4ec70bb3780b0bb 100644
--- a/dune/solvers/norms/twonorm.hh
+++ b/dune/solvers/norms/twonorm.hh
@@ -35,6 +35,7 @@ class TwoNorm : public Norm<VectorType>
         //! Compute the norm of the difference of two vectors
         virtual double diff(const VectorType& f1, const VectorType& f2) const
         {
+            assert(f1.size() == f2.size());
             double r = 0.0;
 
             for (size_t i=0; i<f1.size(); ++i)