PHP - alguien me ayuda con este error Warning: Illegal string offset "grade_id" y "grade_name" en php

$output = '';

$query = "SELECT * FROM tbl_grade WHERE grade_id = '".$_POST["grade_id"]."'";

$statement = $connect->prepare($query);

if($statement->execute())

{

    $result = $statement->fetchAll();

    foreach($result as $row)

    {

        $output['grade_id'] = $row['grade_id'];

        $output['grade_name'] = $row['grade_name'];

    }

    echo json_encode($output);

}

$output=[]

$output=array();