Recurring Select Accepts "Null" As a Valid Rule

I followed the Chris' excellent tutorial on Recurring events with the ice_cube gem. However, I was running into an issue when the value of recurring was set to - not recurring -. If I went to create or update an event, I would get the following error:

undefined method 'to_hash' for nil:NilClass, which pointed to this line super(RecurringSelect.dirty_hash_to_rule(value).to_hash)

The actual value of this field is null, and for some reason this value was being accepted as a RecurringSelect.is_valid_rule?.

I ended up rewriting my recurring= method to the following to solve the issue.

  def recurring=(value)
    if value == "null"
      super(nil)
    elsif RecurringSelect.is_valid_rule?(value)
      super(RecurringSelect.dirty_hash_to_rule(value).to_hash)
    else
      super(nil)
    end
  end

1. Is my solution acceptable?
2. Any reason why I'm running into this issue? Is it because I'm running a different version of Rails? I'm on rails 5