Migration problem in rail 2.0 and observers

I have a somewhat confusing error !!
I am using ruby 1.8.7 rails 2.0 and text mate and have observers that
are defined in cofiguration file as
config.active_record.observers = :flock_observer, :text_observer
I cannot use migrate when this configuration is active, I had to delete
it to be able to migrate. After the migration I add the configuration
again and all is OK!
What could be the reason for that behavior ?
How could one fix it ?