Adding a chaining .not() custom assertion for nightwatch.js

I'm using nightwatch.js to do some ui integration tests and I didn't find any functionality that allows me to assert for the opposite of an expectation. I remember I used to have a .not() method in jasmine.js and I am looking for that kind of functionality in nightwatch as well. I couldn't find any custom methods online and I'm having difficulties developing a custom assertion myself.

My goal is to do: Browser.assert.not().title("Login").end();