describe('the fahrenheit2celsius canary spec', () => {
  it('shows the infrastructure works', () => {
    true.should.be.true();
  });

  describe('fahrenheit2celsius should', () => {
    it('give 0°C for 32°F', () => {
      fahrenheit2celsiusOf(32).should.equal(0);
    });
    it('give 10°C for 50°F');
  });
});

describe('the fahrenheit2celsius canary spec', () => {
  it('shows the infrastructure works', () => {
    true.should.be.true();
  });

  let fahrenheit2celsiusOf = function(number) {
    
  }
  describe('fahrenheit2celsius should', () => {
    it('give 0°C for 32°F', () => {
      fahrenheit2celsiusOf(32).should.equal(0);
    });
    it('give 10°C for 50°F');
  });
});

change message to TypeError: Cannot read property 'should' of undefined