diff --git a/ChangeLog b/ChangeLog
index 7a229ff735..5c5939531e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sun Nov 13 10:23:48 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* bignum.c (rb_big2ull): fix off-by-twice bug of NUM2ULL.
+	* test/-ext-/num2int/test_num2int.rb (class TestNum2int):
+	  fix a testcase too.
+
 Sun Nov 13 10:22:44 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* test/-ext-/num2int/test_num2int.rb (class TestNum2int):
diff --git a/bignum.c b/bignum.c
index 7bcc4c7935..91c275e605 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1257,8 +1257,14 @@ rb_big2ull(VALUE x)
 {
     unsigned LONG_LONG num = big2ull(x, "unsigned long long");
 
-    if (!RBIGNUM_SIGN(x))
-	return (VALUE)(-(SIGNED_VALUE)num);
+    if (!RBIGNUM_SIGN(x)) {
+	VALUE v = (VALUE)(-(SIGNED_VALUE)num);
+
+	/* FIXNUM_MIN-1 .. LLONG_MIN mapped into 0xbfffffffffffffff .. LONG_MAX+1 */
+	if (v <= LLONG_MAX)
+	    rb_raise(rb_eRangeError, "bignum out of range of unsigned long long");
+	return v;
+    }
     return num;
 }
 
diff --git a/test/-ext-/num2int/test_num2int.rb b/test/-ext-/num2int/test_num2int.rb
index 4e2dc37bfc..b27606288c 100644
--- a/test/-ext-/num2int/test_num2int.rb
+++ b/test/-ext-/num2int/test_num2int.rb
@@ -162,20 +162,11 @@ class TestNum2int < Test::Unit::TestCase
     assert_output(ULLONG_MAX.to_s) do
       Num2int.print_num2ull(-1)
     end
-    assert_output((LLONG_MAX+2).to_s) do
-      Num2int.print_num2ull(LLONG_MIN+1)
-    end
-
-    # maybe bug
-    assert_output((LLONG_MAX).to_s) do
-      Num2int.print_num2ull(LLONG_MIN-1)
-    end
-    # maybe bug
-    assert_output(1.to_s) do
-      Num2int.print_num2ull(LLONG_MIN*2+1)
+    assert_output((LLONG_MAX+1).to_s) do
+      Num2int.print_num2ull(LLONG_MIN)
     end
     assert_raise(RangeError) do
-      Num2int.print_num2ull(LLONG_MIN*2)
+      Num2int.print_num2ull(LLONG_MIN-1)
     end
     assert_raise(RangeError) do
       Num2int.print_num2ull(ULLONG_MAX+1)