problems to create a json view (rest export) in Drupal 8, select count(*) from content type = "..." and published = yes

I have problems to create a json view (rest export) in Drupal 8, my goal : select count(*) from content type = "..." and published = yes
The output must be json notation and the total amount of nodes for a specific content type.
In the Advanced selection I set "use aggregation = yes"
under Fields I use the aggregation setting "count" on content type
Filter criteria: published status = yes, content type = myCT

Result: [{"type":"1"},{"type":"1"},{"type":"1"},....]

I did the same in drupal 7 without any issues but can't get it work in D8.
Has anybody an idea how to get this solved