diff --git a/libsrc/core/mpi_wrapper.hpp b/libsrc/core/mpi_wrapper.hpp
index 4928dfda..329fee57 100644
--- a/libsrc/core/mpi_wrapper.hpp
+++ b/libsrc/core/mpi_wrapper.hpp
@@ -153,7 +153,7 @@ namespace ngcore
     /** --- collectives --- **/
 
     template <typename T, typename T2 = decltype(GetMPIType<T>())> 
-    T Reduce (T d, const MPI_Op & op, int root = 0)
+    T Reduce (T d, const MPI_Op & op, int root = 0) const
     {
       if (size == 1) return d;