value is not stored in database when datatype is bigint

[vinnyjismy]

i have created a database field for storing the checkbox value. The value say 2,3 is saved into database as expected when field type is text. But when this field type is Bigint the single value is only get saved even if i select multiple checkbox ie only saving value 2 instead of 2,3 in database
$options = [];
$groups = Group::select('id', 'name')->orderBy('name')->get();

    foreach($groups as $group) {
        $options[$group->id] = $group->name;
    }
   Checkboxes::make('groups')->options($options)->saveAsString(),