diff --git a/ChangeLog b/ChangeLog
index 9104faeb1a..cab8941d1a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jan  5 05:04:39 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* gc.c (vm_xrealloc): add a few comment why we avoid realloc(ptr,0).
+
 Fri Jan  4 20:17:06 2013  Yuki Yugui Sonoda  <yugui@yugui.jp>
 
 	* Makefile.in (RBCONFIG): Moved from common.mk in order to use the
diff --git a/gc.c b/gc.c
index c79fd73dc9..341863d8d4 100644
--- a/gc.c
+++ b/gc.c
@@ -3529,7 +3529,14 @@ vm_xrealloc(rb_objspace_t *objspace, void *ptr, size_t size)
     if ((ssize_t)size < 0) {
 	negative_size_allocation_error("negative re-allocation size");
     }
+
     if (!ptr) return vm_xmalloc(objspace, size);
+
+    /*
+     * The behavior of realloc(ptr, 0) is implementation defined.
+     * Therefore we don't use realloc(ptr, 0) for portability reason.
+     * see http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_400.htm
+     */
     if (size == 0) {
 	vm_xfree(objspace, ptr);
 	return 0;