diff --git a/cpp/src/phonenumbers/base/memory/singleton.h b/cpp/src/phonenumbers/base/memory/singleton.h
index 43c5a1aa6..38b4bfc4e 100644
--- a/cpp/src/phonenumbers/base/memory/singleton.h
+++ b/cpp/src/phonenumbers/base/memory/singleton.h
@@ -50,7 +50,8 @@ template <class T> boost::once_flag Singleton<T>::flag = BOOST_ONCE_INIT;
 
 #else  // !I18N_PHONENUMBERS_USE_BOOST
 
-#include "phonenumbers/base/thread_safety_check.h"
+#include "phonenumbers/base/logging.h"
+#include "phonenumbers/base/thread_checker.h"
 
 namespace i18n {
 namespace phonenumbers {
@@ -60,6 +61,8 @@ namespace phonenumbers {
 template <class T>
 class Singleton {
  public:
+  Singleton() : thread_checker_() {}
+
   virtual ~Singleton() {}
 
   static T* GetInstance() {
@@ -67,8 +70,12 @@ class Singleton {
     if (!instance) {
       instance = new T();
     }
+    DCHECK(instance->thread_checker_.CalledOnValidThread());
     return instance;
   }
+
+ private:
+  const ThreadChecker thread_checker_;
 };
 
 #endif  // !I18N_PHONENUMBERS_USE_BOOST
diff --git a/cpp/src/phonenumbers/base/synchronization/lock.h b/cpp/src/phonenumbers/base/synchronization/lock.h
index c68fefd60..7ef136c20 100644
--- a/cpp/src/phonenumbers/base/synchronization/lock.h
+++ b/cpp/src/phonenumbers/base/synchronization/lock.h
@@ -30,17 +30,35 @@ typedef boost::mutex::scoped_lock AutoLock;
 }  // namespace i18n
 
 #else  // I18N_PHONENUMBERS_USE_BOOST
-#include "phonenumbers/base/thread_safety_check.h"
+
+#include "phonenumbers/base/logging.h"
+#include "phonenumbers/base/thread_checker.h"
 
 namespace i18n {
 namespace phonenumbers {
 
 // Dummy lock implementation. If you care about thread-safety, please compile
 // with -DI18N_PHONENUMBERS_USE_BOOST.
-struct Lock {};
+class Lock {
+ public:
+  Lock() : thread_checker_() {}
+
+  void Acquire() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+  }
+
+  // No need for Release() since Acquire() is a no-op and Release() is not used
+  // in the codebase.
+
+ private:
+  const ThreadChecker thread_checker_;
+};
 
-struct AutoLock {
-  AutoLock(Lock) {}
+class AutoLock {
+ public:
+  AutoLock(Lock& lock) {
+    lock.Acquire();
+  }
 };
 
 }  // namespace phonenumbers
diff --git a/cpp/src/phonenumbers/base/thread_safety_check.h b/cpp/src/phonenumbers/base/thread_checker.h
similarity index 66%
rename from cpp/src/phonenumbers/base/thread_safety_check.h
rename to cpp/src/phonenumbers/base/thread_checker.h
index b11bdf576..38ef54c26 100644
--- a/cpp/src/phonenumbers/base/thread_safety_check.h
+++ b/cpp/src/phonenumbers/base/thread_checker.h
@@ -28,4 +28,41 @@
 
 #endif
 
+#if !defined(NDEBUG) && !defined(I18N_PHONENUMBERS_USE_BOOST) && \
+    (defined(__linux__) || defined(__apple__))
+
+#include <pthread.h>
+
+namespace i18n {
+namespace phonenumbers {
+
+class ThreadChecker {
+ public:
+  ThreadChecker() : thread_id_(pthread_self()) {}
+
+  bool CalledOnValidThread() const {
+    return thread_id_ == pthread_self();
+  }
+
+ private:
+  const pthread_t thread_id_;
+};
+
+#else
+
+namespace i18n {
+namespace phonenumbers {
+
+class ThreadChecker {
+ public:
+  bool CalledOnValidThread() const {
+    return true;
+  }
+};
+
+#endif
+
+}  // namespace phonenumbers
+}  // namespace i18n
+
 #endif  // I18N_PHONENUMBERS_BASE_THREAD_SAFETY_CHECK_H_