diff --git a/include/boost/mpi/collectives/scan.hpp b/include/boost/mpi/collectives/scan.hpp
index 4cba4a8..9264838 100644
--- a/include/boost/mpi/collectives/scan.hpp
+++ b/include/boost/mpi/collectives/scan.hpp
@@ -47,7 +47,7 @@ namespace detail {
   template<typename T, typename Op>
   void
   scan_impl(const communicator& comm, const T* in_values, int n, T* out_values,
-            Op op, mpl::true_ /*is_mpi_op*/, mpl::true_ /*is_mpi_datatype*/)
+            Op /*op*/, mpl::true_ /*is_mpi_op*/, mpl::true_ /*is_mpi_datatype*/)
   {
     BOOST_MPI_CHECK_RESULT(MPI_Scan,
                            (const_cast<T*>(in_values), out_values, n,