diff --git a/tests/test_utility.cpp b/tests/test_utility.cpp
index 1512e4f..dd08f2f 100644
--- a/tests/test_utility.cpp
+++ b/tests/test_utility.cpp
@@ -30,7 +30,7 @@ BOOST_AUTO_TEST_CASE(test_resize)
             thrown = true;
         }
         BOOST_CHECK(!thrown);
-        BOOST_CHECK_EQUAL(v.size(), 10);
+        BOOST_CHECK_EQUAL(v.size(), 10u);
     }
 
     {
@@ -45,7 +45,7 @@ BOOST_AUTO_TEST_CASE(test_resize)
             thrown = true;
         }
         BOOST_CHECK(!thrown);
-        BOOST_CHECK_EQUAL(a.size(), 15);
+        BOOST_CHECK_EQUAL(a.size(), 15u);
     }
 
     {
@@ -62,3 +62,9 @@ BOOST_AUTO_TEST_CASE(test_resize)
         BOOST_CHECK(thrown);
     }
 }
+
+BOOST_AUTO_TEST_CASE(test_concat_to_string)
+{
+    const std::string cat = toml::concat_to_string("foo", "bar", 42);
+    BOOST_CHECK(cat == "foobar42");
+}