Require JS oddness

gistfile1.js

// this works
define(function (require) {
    var a = require('a');
    return a;
});

// this fails, and I have no idea why
define(function (require) {
    var a = 'a';
    var b = require(a);
    return b;
});

// this also fails
define(function (require) {
    var a = require('a'+'');
    return a;
});