diff --git a/ChangeLog b/ChangeLog
index 87f992ca47..b9a6b6a9e3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri May 28 11:40:07 2010  URABE Shyouhei  <shyouhei@ruby-lang.org>
+
+	* array.c (rb_ary_product): Do not rely on GC, t0 should be
+	  checked explicitly.
+
 Fri May 28 10:40:37 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (checking_for): ignore toplevel.
diff --git a/array.c b/array.c
index 280f45bba1..146b03b137 100644
--- a/array.c
+++ b/array.c
@@ -4345,10 +4345,14 @@ rb_ary_product(int argc, VALUE *argv, VALUE ary)
 
 	/* put it on the result array */
 	if(NIL_P(result)) {
+	    FL_SET(t0, FL_USER5);
 	    rb_yield(subarray);
-	    if (RBASIC(t0)->klass) {
+	    if (! FL_TEST(t0, FL_USER5)) {
 		rb_raise(rb_eRuntimeError, "product reentered");
 	    }
+	    else {
+		FL_UNSET(t0, FL_USER5);
+	    }
 	}
 	else {
 	    rb_ary_push(result, subarray);