How to update an array in Meteor

I’m trying to add an element in an array but can’t figure out what is causing my problem,
Don’t know if i have to do an insert or an update anyway tried both, both not working

  check(_id, String)
  check(email, String)
  var current_test = Modules.test.checkTestExist(test_id);
  const USER = Accounts.findUserByEmail(email)
  let test=USER._id
    Tests.update(_id, {$set: {'name': name}}) // this one getting updated
    Tests.update(test_id, { $set: { "current_test.senders": test} }) // this is not working 

i got the error

UserIds that can send messages on this test must be an array",

Here’s an example that may help.

        _id: test_id,
        current_test: { $elemMatch: { senders: oldTestValue } }
      }, {
        $set: {
          'current_test.$.senders': newTestValue
      }, { multi: true });