Is Zakat obligatory on a house that was bought four years ago and rented out a year ago, but has not generated income after covering maintenance expenses?

Zakat is not obligatory on the value of property designated for rent, but rather on its income (rent) if it reaches the [nisab] threshold and a full year [hawl] passes over it. If the income for the year is spent on maintaining the house, then there is no Zakat due on it for that year.