child_process module not returning directory

I'm doing this tutorial on NodeJS: http://www.nodebeginner.org.

Here is the code I find confusing:

var exec = require("child_process").exec;

    function start(response) {
        console.log("Request handler 'start' was called.");
        var content = "empty";

        exec("ls -lah", function(error, stdout, stderr) {
            response.writeHead(200, {"Content-type":"text/plain"});
            response.write(stdout);
            console.log(stdout);
            response.end();
        });

    }

I have a router that passes the http response to a request handler that calls the start function. This is happening without a problem. However, the stdout parameter is not returning anything in the browser or in the console. I understand that ls -lah is supposed to give a list of files in the current directory. I have 5 other files in my directory, but nothing is being returned. Any ideas on what is happening here?