That's a mistake, your answer is correct. The stem is indeed "houd", and the formal correct answer here is "Ik houd...". However, there is practically no one in The Netherlands that says or writes that. We say "Ik hou van mijn hond".
(Not to be confused with the same verb in the different meaning "keeping", it can only be "Ik houd mijn hond" -> I keep my dog)