Can't get morgan logger to only print failed requests

I'm trying to get morgan logger to only print failed requests. The docs (https://www.npmjs.org/package/morgan) say to do this:

// EXAMPLE: only log error responses
morgan('combined', {
    skip: function (req, res) { return res.statusCode < 400 }
})

So here's my code:

var express = require('express');
var morgan = require('morgan');

var app = express();

app.use(morgan('combined', {
    skip: function(req, res) { return res.statusCode < 400 }
}));

However, every request just prints out "combined", and nothing else. Am I misunderstanding their example? I don't really see what else I should be doing.