PHP SyntaxError: Unexpected end of JSON input

I'm building an App with React and Mobx which gets served data as JSON from an PHP/MySQL backend. The MySQL-table until now contained only simple varchar-fields and so far it worked ok. Now I added a Date-field to the table, and now I get the error SyntaxError: Unexpected end of JSON input- Why is that?

The PHP Code looks like this:

$app->get('/api/releases', function(Request $request, Response $response) {
$sql = "SELECT * FROM albums";

try{
    $db = new db();

    $db = $db->connect();

    $stmt = $db->query($sql);
    $releases = $stmt->fetchAll(PDO::FETCH_OBJ);
    $db = null;
    echo json_encode($releases);
   } catch(PDOException $e) {
       echo '{"error": {"text": '.$e->getMessage().'}';
     }
});

Should the Date be converted to a string first?