Record was saved to the server, but the response returned the new id 'null'. The store cannot assign a new id to a record that already has an id

Ember Data
3

My response from the backend looks like this:

For findAll:

[
{
    "_id": "5cdb30ca2fcde3c9ba7974ce",
    "title": "Data",
    "__v": 0
},
{
    "_id": "5cdb3109aeb661cab4cf344c",
    "title": "Purple",
    "__v": 0
},
{
    "_id": "5cdb31fa85b86acd9876137f",
    "title": "New id",
    "__v": 0
}
]

And a single response:
{
"_id": "5cdb30ca2fcde3c9ba7974ce",
"title": "Data",
"__v": 0
}