TypeError: 'undefined' is not a function (evaluating 'sinon.spy()')

I'm trying to use sinon.js in testing of a backbone application. But unfortunately I cannot use spy method due to error:

TypeError: 'undefined' is not a function (evaluating 'sinon.spy()')

Here is the steps to reproduce the error:

1. Create an empty project with backbone yeoman generator
2. Install sinon: cd test && bower install sinon
3. Include in test/index.html <script src="bower_components/sinon/lib/sinon.js"></script>

4. Create spy in test/spec/test.js:

   describe('Give it some context', function () {
       describe('maybe a bit more context here', function () {
           it('should run here few assertions', function () {
             var spy = sinon.spy();
             spy.should.be.ok;
           });
       });
   });
   

5. Run the test with grunt: grunt test

6. The test will fail with a described error.

Could anyone help to find out what is wrong?