Posts by category return wrong result

Hi,
Im trying to get the posts by category. using this URL:
/api/core/get_category_posts/?slug=cat2
But responding this:

{“status”:”ok”,”count”:0,”pages”:0,”category”:{“id”:3,”slug”:”cat2″,”title”:”Cat2″,”description”:””,”parent”:0,”post_count”:7},”posts”:[]}

Posts are empty! any idea?

https://wordpress.org/plugins/json-api/