diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index b4f1df7a8..40c4f5835 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -63,7 +63,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
                 {
                     if (experimentalFeatureSettings.isEnabled(Xp::ParseTomlTimestamps)) {
                         auto attrs = state.buildBindings(2);
-                        attrs.alloc("_type").mkString("timestamp");
+                        attrs.alloc("_type").mkStringNoCopy("timestamp");
                         std::ostringstream s;
                         s << t;
                         attrs.alloc("value").mkString(s.str());
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index a24dd11d6..3a1ebc2ed 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -61,7 +61,7 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
 
         auto attrs = state.buildBindings(7 + outputs.size());
 
-        attrs.alloc(state.sType).mkString("derivation");
+        attrs.alloc(state.sType).mkStringNoCopy("derivation");
         attrs.alloc(state.sName).mkString(i.queryName());
         auto system = i.querySystem();
         if (!system.empty())