diff --git a/dune/istl/matrixmatrix.hh b/dune/istl/matrixmatrix.hh
index 57e78ef902c270ea912f0baf1703a1d522070c9c..2b100c706878c65e73892614cd4c677337605eab 100644
--- a/dune/istl/matrixmatrix.hh
+++ b/dune/istl/matrixmatrix.hh
@@ -103,7 +103,7 @@ namespace Dune
                            F& func)
       {
         if(mat.M()!=matt.M())
-          DUNE_THROW(ISTLError, "The sizes of the matrices do not match: "<<mat.N()<<"!="<<matt.N());
+          DUNE_THROW(ISTLError, "The sizes of the matrices do not match: "<<mat.M()<<"!="<<matt.M());
 
         typedef typename BCRSMatrix<FieldMatrix<T,n,m>,A1>::ConstRowIterator row_iterator;
         typedef typename BCRSMatrix<FieldMatrix<T,n,m>,A1>::ConstColIterator col_iterator;