diff --git a/fem/discreteoperatorimp.hh b/fem/discreteoperatorimp.hh
index 22f4a078bfe6ec65390de86409b304777d847c48..8e72886bcd9da35f5137e8717a603ed2b96215e9 100644
--- a/fem/discreteoperatorimp.hh
+++ b/fem/discreteoperatorimp.hh
@@ -108,7 +108,7 @@ namespace Dune {
       typedef typename DFDomainType::FunctionSpace FunctionSpaceType;
       typedef typename FunctionSpaceType::GridType GridType;
       // the corresponding grid
-      FunctionSpaceType & functionSpace_= dest.getFunctionSpace();
+      const FunctionSpaceType & functionSpace_= dest.getFunctionSpace();
       GridType &grid = functionSpace_.getGrid();
 
       if(leaf_)