diff --git a/ChangeLog b/ChangeLog
index 959b1286bf..48902b03b8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Mon Apr  1 21:10:56 2013  Tanaka Akira  <akr@fsij.org>
+
+	* numeric.c (rb_num2ulong_internal): Don't cast a negative double value
+	  into unsigned long, which is undefined behavior.
+	  (rb_num2ull): Don't cast a value bigger than LLONG_MAX into
+	  long long, which is undefined behavior.
+
 Mon Apr  1 20:57:57 2013  Tanaka Akira  <akr@fsij.org>
 
 	* ext/-test-/num2int/num2int.c: Return string for result, instead of
diff --git a/numeric.c b/numeric.c
index a4eb0dd167..66820eeed1 100644
--- a/numeric.c
+++ b/numeric.c
@@ -2001,7 +2001,9 @@ rb_num2ulong_internal(VALUE val, int *wrap_p)
            double d = RFLOAT_VALUE(val);
            if (wrap_p)
                *wrap_p = d <= -1.0; /* NUM2ULONG(v) uses v.to_int conceptually.  */
-           return (unsigned long)d;
+           if (0 <= d)
+               return (unsigned long)d;
+           return (unsigned long)(long)d;
        }
        else {
            char buf[24];
@@ -2273,6 +2275,8 @@ rb_num2ull(VALUE val)
       case T_FLOAT:
 	if (RFLOAT_VALUE(val) < ULLONG_MAX_PLUS_ONE
             && LLONG_MIN_MINUS_ONE_IS_LESS_THAN(RFLOAT_VALUE(val))) {
+            if (0 <= RFLOAT_VALUE(val))
+                return (unsigned LONG_LONG)(RFLOAT_VALUE(val));
 	    return (unsigned LONG_LONG)(LONG_LONG)(RFLOAT_VALUE(val));
 	}
 	else {