diff --git a/src/optimizing-compiler-thread.cc b/src/optimizing-compiler-thread.cc
index b9ff7d83ec..dbf9ad7cdc 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -39,7 +39,9 @@ namespace internal {
 
 void OptimizingCompilerThread::Run() {
 #ifdef DEBUG
-  thread_id_ = ThreadId::Current().ToInteger();
+  { ScopedLock lock(thread_id_mutex_);
+    thread_id_ = ThreadId::Current().ToInteger();
+  }
 #endif
   Isolate::SetIsolateThreadLocals(isolate_, NULL);
   DisallowHeapAllocation no_allocation;
@@ -156,6 +158,7 @@ void OptimizingCompilerThread::QueueForOptimization(
 #ifdef DEBUG
 bool OptimizingCompilerThread::IsOptimizerThread() {
   if (!FLAG_parallel_recompilation) return false;
+  ScopedLock lock(thread_id_mutex_);
   return ThreadId::Current().ToInteger() == thread_id_;
 }
 #endif
diff --git a/src/optimizing-compiler-thread.h b/src/optimizing-compiler-thread.h
index 004fce7ada..59c94cb214 100644
--- a/src/optimizing-compiler-thread.h
+++ b/src/optimizing-compiler-thread.h
@@ -46,6 +46,7 @@ class OptimizingCompilerThread : public Thread {
       Thread("OptimizingCompilerThread"),
 #ifdef DEBUG
       thread_id_(0),
+      thread_id_mutex_(OS::CreateMutex()),
 #endif
       isolate_(isolate),
       stop_semaphore_(OS::CreateSemaphore(0)),
@@ -89,6 +90,7 @@ class OptimizingCompilerThread : public Thread {
  private:
 #ifdef DEBUG
   int thread_id_;
+  Mutex* thread_id_mutex_;
 #endif
 
   Isolate* isolate_;