Good question. It sounded strange to me so I told my friend, and it turns out that the way the sentence is pronounced is correct here, but if you check the word дома itself, it reads it damA, meaning "houses". So the accent (remembering that unstressed o changes into a) changes the meaning of the word actually...
This particular word is an adverb, stuck with one specific form (дома "at home"). For everything else, you use prepositions:
- Я на работе. = I am at work.
- Я в доме. = I am in the house.
- Я в школе. = I am in the school.
- на концерте = at a concert
- на станции = at a station
- в математике = in mathematics
We also have a different directional adverb домой "(to) home". It behaves similar to the English "home" in expressions like "go home".