userId parameter is missing for message sent to a group?

Question

Normally when someone send a message to a group where my BOT is in, my BOT will received response of event object consists of common properties like following:
  "source": {
    "type": "group",
    "groupId": "Ca56f94637c...",
    "userId": "U4af4980629..."
  }
	
However, I found the parameter "userId" is missing for some users in the same group. my BOT only received:
  "source": {
    "type": "group",
    "groupId": "Ca56f94637c...",
  }

how it could possible?
Thank you



Product: Messaging API
App type :BOT
Plan :Free
Available features: REPLY_MESSAGE

Serrated &C · Accepted Answer

Users who are have not accepted the LINE Official Account Terms Of Use (which most will find by friending the bot) will not have their user ID appear when the bot is notified of messages.

https://developers.line.me/en/docs/messaging-api/user-consent/

Q&A

userId parameter is missing for message sent to a group?