Cannot construct with a IP network prefix as README.md suggests

[kwpeters]

README.md states that one of the ways a Netmask can be constructed is with the following parameters:

'216.240.32.0', 0xffffff00

The following code, however, causes an Error to be thrown:

var netmask = require("netmask");
var block = new netmask.Netmask('10.0.0.0', 0xffffff00);

The code that throws the error is here.