An appropriate question tag for "He's too weak to walk."?

Question

A couple of days ago, I sat for an English exam. There was a question there that asked for the appropriate question tag for the sentence "He's too weak to walk." I answered "isn't he?" but my teacher claimed that the correct answer should be "is he?" since the sentence expresses a negative meaning. Frankly his explanation made no sense to me. Is he right? Am I missing something? What should be the appropriate question tag for the sentence *He's too weak to walk.*? ----- When we're asked to add question tags, we are expected to add tags that agree with the assertion, for example, *It's a great day, isn't it?* Question tags that genuinely express doubt whether the sentence is true or not aren't expected.

Answer 1

I agree with you.

He is too weak to walk or is he not? = He is too weak to walk, isn't he?

According to the grammar a positive statement is followed by a negative tag. More a grammar questions than content in my opinion.