What's wrong with using 内に in this sentence?

Question

My textbook claims that this is an incorrect usage of uchi ni:

彼が会社に行っている内に、贈り物が届きました
While he is commuting to the office, a present arrived.

Why is this wrong? It looks fine to me.

Answer 1

(Updated response)

I agree with Istrasci that 間に　is probably correct and wonder if, as snailplane suggests, that your textbook might give more context. I'd like to offer an extended solution that you can verify:

うちに is used in the following two cases:

1) A person intentionally takes action before something changes:

eg 日本にいるうちに、一度富士山に登ってみたい。

2) While something is taking place, something changes without the speakers volition (my text book refers to volition. I take it to mean "as a result of the speaker's conscious effort")

An example of 2 would be:

eg 本屋で読みたい本を探しているうちに待ち合わせの時間を過ぎってしまった。
or 気がつかないうちに外は暗くなっていた。

間に can be defined as:

While a continuous action is taking place a momentary action or change occurs.

eg 私が旅行で留守の間に、庭に草がたくさん生えてしまった。

As Istrasci says, 間に also seems to fit. I would add that the sentence certainly does not fit the first definition of うちに and possibly the 間に defintion fits better than the second definition...Is this perhaps a multiple choice question giving both うちに and 間に as possible answers? If so then the question is asking you to choose the better answer.

(Reference: The 完全マスター volume for JLPT N3 文法)

Answer 2

Not 100% positive on this, but I would say 行っている間に. 内に usually indicates "while it is in state X", but implying that state X will eventually change to state Y, and the action is undesirable/impossible in state Y. The example I usually try to remember is

スープが温かいうちに飲んでください　→　Eat (drink) your soup while it's still warm.

because once it cools down, it won't be as delicious; desirable to eat; etc.

So I think 内に doesn't fit because the present could have still been delivered each after his state changed from "going to the office" to another state ("being at the office", "going to lunch", etc.)