Ember CLI: Attempting to register an unknown factory: `controller:object`

My ember-cli version is 0.2.3. I am getting the following error when I try to run tests.

TypeError: Attempting to register an unknown factory: `controller:object`
    at Object.Registry.register (http://localhost:7357/assets/vendor.js:12208:15)
    at Object.container.(anonymous function) [as register] (http://localhost:7357/assets/test-support.js:1905:44)
    at Object.isolatedContainer [as default] (http://localhost:7357/assets/test-support.js:1946:15)
    at exports.default.klassy.Klass.extend._setupIsolatedContainer (http://localhost:7357/assets/test-support.js:2406:52)
    at exports.default.klassy.Klass.extend.setupContainer (http://localhost:7357/assets/test-support.js:2312:14)
    at nextStep (http://localhost:7357/assets/test-support.js:2300:53)
    at exports.default.klassy.Klass.extend.invokeSteps (http://localhost:7357/assets/test-support.js:2305:14)
    at exports.default.klassy.Klass.extend.setup (http://localhost:7357/assets/test-support.js:2275:19)
    at Object.qunit.module.setup (http://localhost:7357/assets/test-support.js:1814:16)
    at runHook (http://localhost:7357/assets/test-support.js:3534:20)

Any idea on what might be causing this? Looking like a ember-cli internal issue.