Office 365 CSOM set multi user field throwing Unknown Error

Question

var Users = [];
    var selectedUsers = [1,2,3,4]//Existing user IDs
    for (var i in selectedUsers) {
        var lkupUser = new SP.FieldLookupValue();
        lkupUser.set_lookupId(selectedUsers[i]);
        Users.push(lkupUser);
    }
    oListItem.set_item('Users', Users);//Multi User field.

    oListItem.set_item('Users', new SP.FieldLookupValue().set_lookupId(selectedUsers[0])); //Works in Office 365

Above code works in SharePoint 2013,2010 but it fails in Office 365 with error:

"SchemaVersion":"15.0.0.0","LibraryVersion":"16.0.5708.1205","ErrorInfo":{
"ErrorMessage":"Unknown Error","ErrorValue":null,"TraceCorrelationId":"683ea59d-f04e-3000-9402-01f84f7b5d54","ErrorCode":-1,"ErrorTypeName":"Microsoft.SharePoint.Client.UnknownError"
}

Thanks in advance.