Getting the unique year value in Laravel

I just wanted to extract unique year value from this date type column. But I always get this error "You might need to add explicit type casts".

My desired output should be just "2020" value since I wanted a unique result. Please help. Thank you.

Here is my code:

$year= DB::table('loans')
            ->select('date_release', DB::raw('YEAR(date_release) as year'))
            ->groupBy('year')
            ->get();

via Chebli Mohamed