EmberJS no model was found but it's there

I'm writing an ember-cli app. I have the following model:

// app/models/school.js
export default DS.Model.extend({
  name: DS.attr('string', { defaultValue: '' })
});

It was generated using an ember generator like all my other models. It has a functioning unit test as well that tests for the default value of it's name attribute. All tests are green until another model belongs to school like this:

// app/models/professor.js
export default DS.Model.extend({
  name: DS.attr('string', { defaultValue: '' }),
  email: DS.attr('string', { defaultValue: '' }),

  courses: DS.hasMany('course'),
  posts: DS.hasMany('post'),
  school: DS.belongsTo('school')
});

This test is totally green until I add the school attribute. It's even green with 'model:school' defined in the needs array of the moduleForModel helper:

// tests/unit/models/professor-test.js
// this is green w/o belongsTo('school') in the model
moduleForModel('professor', {
  // Specify the other units that are required for this test.
  needs: ['model:school', 'model:post', 'model:course']
});

The error I'm getting is:

Error: No model was found for 'school'

Here's the models directory

$ ls app/models/
course.js  post.js  professor.js  school.js  student.js

Why is it not finding my model?