在云开发控制台仿照文档command.expr例1测试执行失败！

https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/database/command/Command.expr.html

显示如下错误：RuntimeError: cannot read property 'expr' (from line 1, col 35 to line 1, col 39) > 1 | db.collection('boxcards').where(_.expr($.gt('$a', '$visible'))).get() | ^^^^