stripos() expects parameter 1 to be string, object given laravel - php

Question

code
$data = Event::with(['favourite' => function ($q){
$q->select('*');
$q->groupBy(function ($date) {
return Carbon::parse($date->created_at)->format('M'); **// this line have error**
});
}])
->select('id', 'title')
->get()
->where('id', $eventid)
->toArray();
//if not group by the data then the output will become
[0] => Array
(
[id] => 3
[title] => Everything You Need to Know About Business From Start-up to IPO, Straight From the VC's Mouth
[favourite] => Array
(
[0] => Array
(
[created_at] => 2020-09-10T17:54:06.000000Z
)
[1] => Array
(
[created_at] => 2020-09-10T18:21:00.000000Z
)
[2] => Array
(
[created_at] => 2020-09-28T15:05:38.000000Z
)
[3] => Array
(
[created_at] => 2020-08-28T15:05:38.000000Z
)
)
)
Final data should be
[0] => Array
(
[id] => 3
[title] => Everything You Need to Know About Business From Start-up to IPO, Straight From the VC's Mouth
[favourite] => Array
(
[Aug] => Array
(
[total] => 1
)
[Sep] => Array
(
[total] => 3
)
)
)
Anyone facing this error before? When I try to separate the data as a group. If will return error stripos() expects parameter 1 to be string, object given. The above code will return all the data from database.