community.inviteUserToGroup doen't work in my code

[root]

Hey, 

I want to invite the User if he send the command !invite. 
How can I fix it? 

Log: 

"invitee": userID.toString(),

TypeError: Cannot read property 'toString' of null

if (message == "!invite") {

        community.inviteUserToGroup(null, "XXXXXXXXXXXXXXX");

        console.log("Invite send to " + steamID);

[Dr. McKay]

Why are you passing null?