diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
index 373381f8b..08d4d08ed 100644
--- a/src/fish_tests.cpp
+++ b/src/fish_tests.cpp
@@ -1545,7 +1545,7 @@ static void test_lru() {
         auto commajoin = [](const std::vector<int> &vs) {
             wcstring ret;
             for (int v : vs) {
-                ret.append(std::to_wstring(v));
+                append_format(ret, L"%d,", v);
             }
             if (!ret.empty()) ret.pop_back();
             return ret;