diff --git a/src/lib/util/threads/sync.h b/src/lib/util/threads/sync.h
index cdbd5c1b4f..a539f45fad 100644
--- a/src/lib/util/threads/sync.h
+++ b/src/lib/util/threads/sync.h
@@ -127,6 +127,7 @@ public:
     /// \todo Disable in non-debug build
     bool locked() const;
 
+private:
     /// \brief Lock the mutex
     ///
     /// This method blocks until the mutex can be locked.
diff --git a/src/lib/util/threads/tests/lock_unittest.cc b/src/lib/util/threads/tests/lock_unittest.cc
index 9deddd7fcf..9c17e6fc65 100644
--- a/src/lib/util/threads/tests/lock_unittest.cc
+++ b/src/lib/util/threads/tests/lock_unittest.cc
@@ -26,24 +26,6 @@ using namespace isc::util::thread;
 
 namespace {
 
-TEST(MutexTest, direct) {
-    Mutex mutex;
-    EXPECT_FALSE(mutex.locked()); // Debug-only build
-
-    mutex.lock();
-    EXPECT_TRUE(mutex.locked()); // Debug-only build
-
-    EXPECT_FALSE(mutex.tryLock());
-
-    mutex.unlock();
-    EXPECT_FALSE(mutex.locked()); // Debug-only build
-
-    EXPECT_TRUE(mutex.tryLock());
-
-    mutex.unlock();
-    EXPECT_FALSE(mutex.locked()); // Debug-only build
-}
-
 // If we try to lock the debug mutex multiple times, it should throw.
 TEST(MutexTest, lockMultiple) {
     // TODO: Once we support non-debug mutexes, disable the test if we compile