nginx 返回json错误

	error_page 500 502 503 504 = @503;
    location @503 { 
        # 允许 所有头部 所有域 所有方法
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Headers' '*';
        add_header 'Access-Control-Allow-Methods' '*';
		charset utf-8;
		default_type application/json;
		return 200 '{"code":503,"msg":"系统维护中！","data":null}';
    }