diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index c9ec236c47..91b4f78fa6 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -131,7 +131,7 @@ static inline void mkStringNoCopy(Value & v, const char * s)
 
 static inline void mkString(Value & v, const Symbol & s)
 {
-    mkStringNoCopy(v, ((string) s).c_str());
+    mkStringNoCopy(v, ((const string &) s).c_str());
 }