diff --git a/ChangeLog b/ChangeLog
index 1c817273fa..ae4f8a24aa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Aug 18 20:05:49 2015  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* thread_pthread.c (reserve_stack): ensure the memory is really
+	  allocated. [Bug #11457]
+
 Tue Aug 18 17:19:21 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (IS_BEG): include labeled argument state, which was
diff --git a/thread_pthread.c b/thread_pthread.c
index 0f6e0fdbce..0446501d14 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -686,8 +686,8 @@ reserve_stack(volatile char *limit, size_t size)
 	limit -= size;
 	if (buf > limit) {
 	    limit = alloca(buf - limit);
+	    limit[0] = 0; /* ensure alloca is called */
 	    limit -= stack_check_margin;
-	    limit[0] = 0;
 	}
     }
 }