A question of translation (eorum/earum)

Hi everyone,
I translated this sentence:

The waves, harsher by much, will destroy the trees of the island and their branches.

as follows:

Undae multō asperiōrēs arborēs īnsulae et rāmōs eārum dēlēbunt.

But, in the keys of my work book there was this translation:

Undae multō asperiōrēs arborēs īnsulae et rāmōs eōrum dēlēbunt.

My guess is that it is a mistake, because eārum refers to arbor which is feminine.
Am I wrong?
Thank you.

It should be eārum as you said.

Great, thank you very much.
Happy Easter.