Laravel returning LogicException "must return a relationship instance." - eloquent

Question

I am getting the error whenever the function getCips is being called:
App\Models\Employee::getAllCips must return a relationship instance.
The function that is causing this error is:
private function getCips($user, $page, $search){
if($page == null)$page = 1;
$count = 7;
$cips = null;
if(get_class($user) == User::class)
{
$cips = Visitable::all()->whereNotNull('cip');
}else{
$cips = $user->getAllCips(); //The error is caused because of this line.
}
if($search!=null){
$cips = $cips->filter(function($cip, $i)use($search){
return strpos($cip->name, $search)!==false||strpos($cip->email, $search)!==false||strpos($cip->phone, $search)!==false||strpos($cip->location->name, $search)!==false||strpos($cip->cip->Associated_hospital, $search)!==false;
});
}
$lastPage = ($cips->count()%$count == 0)? intdiv($cips->count(), $count):intdiv($cips->count(), $count)+1;
$page = ($cips->count() == 0)?0:intval($page);
return ["cips"=>$cips->values()->skip(($page-1)*$count)->take($count)->all(),"lastPage"=>$lastPage, "currentPage"=>$page];
}
As marked in the line of code above, the $user is an instance of Employee class.
Now the function getAllCips() in class Employee is as:
public function getAllCips():Collection{
$allCips = $this->headQuarter->getAllCips();
foreach($this->subordinates as $subordinate){
$allCips = $allCips->concat($subordinate->getAllCips);
}
return $allCips;
}
I am using Laravel 8. It would be very helpful if anybody could point out what I am doing wrong here.