diff --git a/src/thread_list.h b/src/thread_list.h
index f5e1a3a..f650174 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -28,7 +28,7 @@
   static const uint32_t kInvalidId = 0;
   static const uint32_t kMainId = 1;
 
-  ThreadList(bool verbose);
+  explicit ThreadList(bool verbose);
   ~ThreadList();
 
   void Dump(std::ostream& os);
@@ -83,7 +83,7 @@
 
 class ThreadListLock {
  public:
-  ThreadListLock(Thread* self = NULL) {
+  explicit ThreadListLock(Thread* self = NULL) {
     if (self == NULL) {
       // Try to get it from TLS.
       self = Thread::Current();