diff --git a/runtime/thread.cc b/runtime/thread.cc
index 269d58e..6114c63 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -1256,7 +1256,7 @@
 
 class CountStackDepthVisitor : public StackVisitor {
  public:
-  CountStackDepthVisitor(Thread* thread)
+  explicit CountStackDepthVisitor(Thread* thread)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
       : StackVisitor(thread, NULL),
         depth_(0), skip_depth_(0), skipping_(true) {}