Jade - Template Engine: How to check if a variable exists

I'm currently using Jade on a new project. I want to render a page and check if a certain variable is available.

app.js:

app.get('/register', function(req, res){
    res.render('register', {
        locals: {
          title: 'Register',
          text: 'Register as a user.',
        }
      });
});

register.jade:

- if (username)
p= username
- else
p No Username!

I always get the following error:

username is not defined

Any ideas on how I can fix this?