HasManyThroughCantAssociateNewRecords Exception

I have a model:
class Video < ActiveRecord::Base
  has_many :video_tags
  has_many :video_types
  has_many :tags, :through => :video_tags, :uniq => :true
  has_many :types, :through => :video_types, :uniq => :true

  def get_tag_list(delimiter)
    tags.collect { |t| t.tag }.join(delimiter)

  def set_tag_list(tag_list, delimiter)
    # doesn't remove old tags
    tag_list.split(',').each do |t|
      tags << Tag.find_or_create_by_tag(t)

When I call
@video.set_tag_list(params[:tag_list], ',')
in my controller, it throws the exception - Cannot associate new records
through 'Video#video_tags' on '#'. Both records must have an id in order
to create the has_many :through record associating them.
However, in the script/console, the method call works just fine. What
could be causing this problem?