diff --git a/src/libutil/tests/tests.cc b/src/libutil/tests/tests.cc
index 35a5d27bb..58df9c5ac 100644
--- a/src/libutil/tests/tests.cc
+++ b/src/libutil/tests/tests.cc
@@ -320,20 +320,15 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(string2Float, emptyString) {
-        double n;
-        ASSERT_EQ(string2Float("", n), false);
+        ASSERT_EQ(string2Float<double>(""), std::nullopt);
     }
 
     TEST(string2Float, trivialConversions) {
-        double n;
-        ASSERT_EQ(string2Float("1.0", n), true);
-        ASSERT_EQ(n, 1.0);
+        ASSERT_EQ(string2Float<double>("1.0"), 1.0);
 
-        ASSERT_EQ(string2Float("0.0", n), true);
-        ASSERT_EQ(n, 0.0);
+        ASSERT_EQ(string2Float<double>("0.0"), 0.0);
 
-        ASSERT_EQ(string2Float("-100.25", n), true);
-        ASSERT_EQ(n, (-100.25));
+        ASSERT_EQ(string2Float<double>("-100.25"), -100.25);
     }
 
     /* ----------------------------------------------------------------------------
@@ -341,20 +336,15 @@ namespace nix {
      * --------------------------------------------------------------------------*/
 
     TEST(string2Int, emptyString) {
-        double n;
-        ASSERT_EQ(string2Int("", n), false);
+        ASSERT_EQ(string2Int<int>(""), std::nullopt);
     }
 
     TEST(string2Int, trivialConversions) {
-        double n;
-        ASSERT_EQ(string2Int("1", n), true);
-        ASSERT_EQ(n, 1);
+        ASSERT_EQ(string2Int<int>("1"), 1);
 
-        ASSERT_EQ(string2Int("0", n), true);
-        ASSERT_EQ(n, 0);
+        ASSERT_EQ(string2Int<int>("0"), 0);
 
-        ASSERT_EQ(string2Int("-100", n), true);
-        ASSERT_EQ(n, (-100));
+        ASSERT_EQ(string2Int<int>("-100"), -100);
     }
 
     /* ----------------------------------------------------------------------------