diff --git a/dune/tectonic/samplefunctional.hh b/dune/tectonic/samplefunctional.hh
index ea9dfceb1b82f988ddfaf5df9fccf11447cc5009..750bba107393d07917242b5970477cb8b99c9c1e 100644
--- a/dune/tectonic/samplefunctional.hh
+++ b/dune/tectonic/samplefunctional.hh
@@ -134,7 +134,7 @@ template <int dim> class SampleFunctional {
 };
 
 template <class Functional>
-void minimise(Functional const J, typename Functional::SmallVector &x,
+void minimise(Functional const &J, typename Functional::SmallVector &x,
               size_t steps, Bisection const &bisection) {
   typedef typename Functional::SmallVector SmallVector;