Please consider the sentence below:
Mr. Moretti has to go to Washington on a business trip unless his associate volunteers to go.
This is a quote from the book: English Grammar Digest, by Trudy Aronson, page 28.
Is this sentence incorrect?
I think the author has meant:
Mr. Moretti has to go to Washington on a business trip unless his associate volunteers [have] to go.
Edit: I think that way because books are at the highest level of confidance for reading and it very rarely happens to see a grammar mistake in a grammar book. Besides, it's the only remedy in my mind (for the time being) to be able to consider the sentence correct.
Do you agree please?