diff --git a/ChangeLog b/ChangeLog
index a6c1bdba1e..0c11f74b05 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jul 21 23:12:53 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* thread_pthread.c (ruby_init_stack): STACK_GROW_DIR_DETECTION is
+	  necessary on platforms with unknown stack direction.  [Bug #6761]
+
 Sat Jul 21 15:13:42 2012  Shota Fukumori  <sorah@tubusu.net>
 
 	* lib/test/unit/testcase.rb(method_added): refactoring.
diff --git a/thread_pthread.c b/thread_pthread.c
index 5d24029c56..be5bbb3947 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -616,6 +616,7 @@ ruby_init_stack(volatile VALUE *addr
 	size_t space = 0;
 #if defined(STACKADDR_AVAILABLE)
 	void* stackaddr;
+	STACK_GROW_DIR_DETECTION;
 	get_stack(&stackaddr, &size);
 	space = STACK_DIR_UPPER((char *)addr - (char *)stackaddr, (char *)stackaddr - (char *)addr);
 #elif defined(HAVE_GETRLIMIT)