diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index c1ffc26fa3..1d4bcd94f7 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -215,7 +215,7 @@ string Settings::pack()
 }
 
 
-void Settings::unpack(string pack) {
+void Settings::unpack(const string &pack) {
     Strings lines = tokenizeString<Strings>(pack, "\n");
     foreach (Strings::iterator, i, lines) {
         string::size_type eq = i->find('=');
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 5e7cfda9a1..13772c65cb 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -25,7 +25,7 @@ struct Settings {
 
     string pack();
 
-    void unpack(string pack);
+    void unpack(const string &pack);
 
     SettingsMap getOverrides();