Populate not working

1

I’ve got items that are in a set:

When the populate param is not present or is 0 I get this in the json

 "set": {
      "_model": "sets",
      "_id": "ddfb97d6366564cfb400018f"
    },

When adding 1 or -1 to to the populate param i get:
"set": null,

All the items are linked to a set.