diff --git a/src/common/Thread.h b/src/common/Thread.h
index 5ab4bfd3c5f..3fc4bc2048c 100644
--- a/src/common/Thread.h
+++ b/src/common/Thread.h
@@ -45,7 +45,7 @@ class Thread {
  public:
   static int get_num_threads(void);
 
-  pthread_t &get_thread_id() { return thread_id; }
+  const pthread_t &get_thread_id() { return thread_id; }
   bool is_started() { return thread_id != 0; }
   bool am_self() { return (pthread_self() == thread_id); }