Bethesda Softworks is having a great year. After announcing and launching the free-to-play mobile version of its “Fallout” series at E3 2015, “Fallout Shelter” apparently made $5.1 million in its first two weeks, according to a report from SuperData Research on Thursday.

The iOS game lets players purchase in-game lunchboxes that contain special items and dwellers, but “Fallout Shelter” is easily played without forking out the extra money, making the earnings very impressive for a mobile game.

“By emphasizing unobtrusive monetization and offline playability, Bethesda managed to earn the respect of a consumer group that is otherwise highly critical of free-to-play monetization,” said the SuperData report.

Originally intended to be a marketing strategy for Bethesda’s “Fallout 4” console and PC game, which will be released on November 10, the company was surprised at the app’s success. “The main thing it has done is give us more faith that the gaming audience is enormous, and they don’t care what device they’re playing on,” said Todd Howard, game director at Bethesda.

Last month “Fallout Shelter” overtook King’s “Candy Crush Saga” in the top-grossing mobile games chart. Bethesda is currently working on an Android version of the app and hopes to release it in the next few months.