Hibernate version: 2.6

I have the tables like in this example:
owner (owner_id, name)
animal (animal_id, owner_id)
cat (animal_id, someattribute)
dog (animal_id, someattribute)

owner can have an collection of dogs and cats.
Animal is abstract thing.

When I try to map like this:

<class name="owner" table="owner">
....
<set name="cats" ...>
<key column="owner_id">
<one-to-many class="Cat"/>
</set>
<set name="dogs" ...>
<key column="owner_id">
<one-to-many class="Dog"/>
</set>
</class>

Then I have an exception
net.sf.hibernate.LazyInitializationException: Failed to lazily initialize a collection ....
Invalid column name 'owner_id'

Do anybody knows how to avoid the problem? I do not want to put owner_id to the cat and dog tables.