I was working through exercise 42 of NH and this one has really got my puzzled. I just can't figure out the grammar of it.
"Relying on his wings Mercurius had no need of a ship."
Key: "Mercurius alis freto nullum nave opus erat."
Nullum doesn't match nave and does 'opus erat' mean he had a need? I really don't get it. To me opus erat would mean that there was work/task.