diff --git a/cpp/etl/etl.cpp b/cpp/etl/etl.cpp
index a98ccd8..c736a48 100644
--- a/cpp/etl/etl.cpp
+++ b/cpp/etl/etl.cpp
@@ -10,7 +10,7 @@ new_score_t transform(const old_score_t &input)
 {
     new_score_t result;
 
-    for (auto pair : input)
+    for (auto const &pair : input)
         for (auto c : pair.second)
             result[tolower(c)] = pair.first;