API error response {"error":"title is missing"} on Issue API call

When using the API to get Issue details got the request as:

[
​{
“iid”: “issue_id”
}​

and the response code is:

​{
“iid” : “Bug ref”,
“state”: “Gitlab state”,
“health_status”: “Gitlab update”,
“web_url”: “URL”,
“milestone”: {
“due_date”: “Due date”,
“state”: “Gitlab state”,
“title”: “version”
}
}​
]

But getting the error - title is missing and not sure what is needed to add to the API to get the milestone associated with that issue. TIA.