Batch: Sequencing requests with the dependsOn property

Question

The documentation Combine multiple requests in one HTTP call using JSON batching for sequencing requests with the dependsOn property indicates that not all calls in the sequence needs to be dependent, however, when making the following batch call I receive the error:

BadRequest - Batch should be either fully sequential or fully parallel

           'requests': [
                {
                  'id': '1',
                  'method': 'GET',
                  'url': '/me/messages?$top=1'
                },
                {
                  'id': '2',
                  'dependsOn': [ '1' ],
                  'method': 'GET',
                  'url': '/me/calendar/events?$top=1'
                },
                {
                  'id': '3',
                  'method': 'GET',
                  'url': 'me/contacts?$top=1'
                }
          ]