diff --git a/src/graph/helper.cc b/src/graph/helper.cc
index 7a929170150fa9d0f4b4db8aed1ba1a3191e0b86..671bec9467f6486b4f30bfb965b9570312a037b3 100644
--- a/src/graph/helper.cc
+++ b/src/graph/helper.cc
@@ -953,7 +953,7 @@ GraphPtr_t graphBuilder(const ProblemSolverPtr_t& ps,
     std::get<2>(objects[i]) = i;
     std::get<1>(objects[i]).resize(od.handles.size());
     Handles_t::iterator it = std::get<1>(objects[i]).begin();
-    for (const std::string hn : od.handles) {
+    for (const std::string& hn : od.handles) {
       *it = robot.handles.get(hn);
       ++it;
     }