diff --git a/src/one-body-sample.cc b/src/one-body-sample.cc
index d41e7db90444599343395e377564221f03c7287c..4ccaa3ca5d19e80fcfb450a90cfee412b1e6f445 100644
--- a/src/one-body-sample.cc
+++ b/src/one-body-sample.cc
@@ -173,8 +173,11 @@ int main() {
     VectorType u2 = u1;
 
     VectorType f(grid.size(grid.maxLevel(), dim));
+    f = 0;
+    VectorType neumannTerm(grid.size(grid.maxLevel(), dim));
     assemble_neumann<GridType, GridType::LeafGridView, SmallVector, P1Basis>(
-        leafView, p1Basis, neumannNodes, f);
+        leafView, p1Basis, neumannNodes, neumannTerm);
+    f += neumannTerm;
 
     // {{{ Assemble terms for the nonlinearity
     std::vector<Dune::FieldVector<double, 1>> nodalIntegrals;