diff --git a/utilities/document/document_db.cc b/utilities/document/document_db.cc
index 8e15a52ca..b9157f274 100644
--- a/utilities/document/document_db.cc
+++ b/utilities/document/document_db.cc
@@ -376,7 +376,7 @@ class IndexKey {
 
 class SimpleSortedIndex : public Index {
  public:
-  SimpleSortedIndex(const std::string field, const std::string& name)
+  SimpleSortedIndex(const std::string& field, const std::string& name)
       : field_(field), name_(name) {}
 
   virtual const char* Name() const override { return name_.c_str(); }