diff --git a/ChangeLog b/ChangeLog
index 179e79fa64..f17357fcae 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Fri Apr 18 17:37:48 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* gc.c (rb_gc_mark_locations): get rid of underflow.
+
+	* gc.c (mark_current_machine_context): check if the main thread stack
+	  position may shink under the intialized position.  [ruby-core:16436]
+
 Thu Apr 17 22:20:52 2008  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* enc/trans/utf_16_32.c (fun_so_to_utf_16be, fun_so_to_utf_16le): add
diff --git a/gc.c b/gc.c
index edcd9b099b..1a826fbc54 100644
--- a/gc.c
+++ b/gc.c
@@ -794,6 +794,7 @@ rb_gc_mark_locations(VALUE *start, VALUE *end)
 {
     long n;
 
+    if (end <= start) return;
     n = end - start;
     mark_locations_array(start,n);
 }
@@ -1491,10 +1492,9 @@ mark_current_machine_context(rb_thread_t *th)
     mark_locations_array((VALUE*)save_regs_gc_mark,
 			 sizeof(save_regs_gc_mark) / sizeof(VALUE));
 
-    mark_locations_array(stack_start, stack_end - stack_start);
+    rb_gc_mark_locations(stack_start, stack_end);
 #ifdef __ia64
-    mark_locations_array(th->machine_register_stack_start,
-			 th->machine_register_stack_end - th->machine_register_stack_start);
+    rb_gc_mark_locations(th->machine_register_stack_start, th->machine_register_stack_end);
 #endif
 #if defined(__human68k__) || defined(__mc68000__)
     mark_locations_array((VALUE*)((char*)STACK_END + 2),