Laravel 5 Invalid argument supplied for foreach()

I'm trying to do a foreach() inside of one of my controller and I'm running into the error of Invalid argument supplied for foreach() this is my code

$subAdmins = $request->input('subAdmin');

  // Adding subAdmin
  foreach($subAdmins as $subAdmin){
    $linksubAdmin = new LayMinisterEvents;
    $linksubAdmin->user_id = $subAdmin;
    $linksubAdmin->event_id = $recordEvent->id;
    $linksubAdmin->save();
 }

This is the data that is getting sent over 'subAdmins' => '5,6'

at HandleExceptions->handleError(2, 'Invalid argument supplied for 
foreach()', '/home/vagrant/www/project-
api/app/Http/Controllers/EventsController.php', 47, array('request' => 
object(Request), 'response' => object(Response), 'authUser' => 1, 
'recordEvent' => object(Events), 'linkadminLeader' => 
object(AdminEvent), 'subAdmins' => '5,6'))
in EventsController.php (line 47)

via Chebli Mohamed