diff --git a/src/libexpr/flake/lockfile.cc b/src/libexpr/flake/lockfile.cc
index a3ed90e1f..e7d77e5a7 100644
--- a/src/libexpr/flake/lockfile.cc
+++ b/src/libexpr/flake/lockfile.cc
@@ -89,7 +89,7 @@ LockFile::LockFile(const nlohmann::json & json, const Path & path)
                 std::string inputKey = i.value();
                 auto k = nodeMap.find(inputKey);
                 if (k == nodeMap.end()) {
-                    auto nodes = json["nodes"];
+                    auto & nodes = json["nodes"];
                     auto jsonNode2 = nodes.find(inputKey);
                     if (jsonNode2 == nodes.end())
                         throw Error("lock file references missing node '%s'", inputKey);