• 当前 Bug 的表现（可附上截图）

WACloud.js:1 Uncaught (in promise) Error: errCode: -404011 cloud function execution error | errMsg: cloud.callFunction:fail requestID 39f6669a-d115-11e8-a18a-52540064d067, cloud function service error code -504002, error message Error: errCode: -502001 database request fail | errMsg: Update Fail: write errors: [{The dollar ($) prefixed field ‘$inc’ in ‘nums.$inc’ is not valid for storage

• 预期表现

正常调用小程序端原子更新没问题，换成云函数端报错

• 复现路径

• 提供一个最简复现 Demo