" strpos() expects parameter 1 to be string, array given " when I make eloquent query

I'm making an web app with Laravel as Api and Vue.js for the front.

I wan't to return a collection of objects but I have an error and I can't fix it.

strpos() expects parameter 1 to be string, array given

public function orderSearch (Request $request) {
        $data = $request->all();
        $aa = $data['search'];
        $orders = Order::whereHas(['customer' => function ($query) use ($aa){
            $query->where('firstname','LIKE','%' . $aa . '%')->orWhere('lastname','LIKE','%' . $aa . '%');
        }])->get();
        return $orders;
    }

my variable $aa is working fine nad my relation between Order and Customer is fine.

Order.php

 public function customer() {
        return $this->belongsTo('App\Model\Customer');
    }

Customer.php

 public function order() {
        return $this->hasMany('App\Model\Order');
    }

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