It's unsigned.

src/supervisor/datatypes.py

@@ -72,7 +72,7 @@ def list_of_exitcodes(arg):
     try:
         vals = list_of_ints(arg)
         for val in vals:
-            if val > 255:
+            if (val > 255) or (val < 0):
                 raise 'Invalid exit code "%s"' % val
         return vals
     except:

src/supervisor/tests/test_datatypes.py

@@ -79,6 +79,7 @@ class DatatypesTest(unittest.TestCase):
         self.assertEqual(vals, [1])
         self.assertRaises(ValueError, datatypes.list_of_exitcodes, 'a,b,c')
         self.assertRaises(ValueError, datatypes.list_of_exitcodes, '1024')
+        self.assertRaises(ValueError, datatypes.list_of_exitcodes, '-1,1')
 
     def test_hasattr_automatic(self):
         datatypes.Automatic