diff --git a/src/libexpr/primops/fetchClosure.cc b/src/libexpr/primops/fetchClosure.cc
index ea6145f6f..4be4dac8f 100644
--- a/src/libexpr/primops/fetchClosure.cc
+++ b/src/libexpr/primops/fetchClosure.cc
@@ -124,7 +124,7 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
     for (auto & attr : *args[0]->attrs()) {
         const auto & attrName = state.symbols[attr.name];
         auto attrHint = [&]() -> std::string {
-            return "while evaluating the '" + attrName + "' attribute passed to builtins.fetchClosure";
+            return fmt("while evaluating the attribute '%s' passed to builtins.fetchClosure", attrName);
         };
 
         if (attrName == "fromPath") {