diff --git a/tests/src/unit-conversions.cpp b/tests/src/unit-conversions.cpp
index b3ab6a8f1..ad6623401 100644
--- a/tests/src/unit-conversions.cpp
+++ b/tests/src/unit-conversions.cpp
@@ -1765,8 +1765,8 @@ TEST_CASE("std::optional")
 {
     SECTION("null")
     {
-        json j_null;
-        std::optional<std::string> opt_null;
+        const json j_null;
+        const std::optional<std::string> opt_null;
 
         CHECK(json(opt_null) == j_null);
         CHECK(j_null.get<std::optional<std::string>>() == std::nullopt);