Rails - Aug 2006 - Save fails due to HATBM relationship... but only if not saved before?

I think this is a pretty weird bug, and would appreciate assistance in
figuring it out.

I have the following model:

class Group < ActiveRecord::Base
	has_and_belongs_to_many :managers, :class_name => "User",
:join_table =>
''groups_managers''
end

When I create a new Group object, assign a manager, and save, I get an
error:

$ ruby script/console
Loading development environment.
>> @group = Group.new(:name => ''test'')
=> #<Group:0x4081b908>
>> @group.managers << User.find(1)
=> [#<User:0x40b9ba10>]
>> @group.save!
ActiveRecord::RecordInvalid: Validation failed: Managers is invalid
        from
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.4/lib/active_record/validations.rb:748:in
`save!''
        from (irb):3

However, if I save the new Group object first, then assign the manager, then
save again, then it works:
>> @group = Group.new(:name => ''test_second'')
=> #<Group:0x40b75b08>
>> @group.save!
=> true
>> @group.managers << User.find(1)
=> [#<User:0x40b55830>]
>> @group.save!
=> true

What could be causing this?  Has anybody ran into this problem before?

Thanks!
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