You might think that a mobile port of a hit PC game could only make someone money. Think again.

Gamasutra has pointed out that Blizzard could actually be losing out on a big chunk of its $20 million a month revenue because of mobile ports of its hit free-to-play card game, Hearthstone: Heroes of Warcraft. Hearthstone makes money from in-game purchases. On the PC version, all of that cash went to Blizzard. But on mobile, Apple and Google take a 30 percent cut.

We reached out to Blizzard and asked them about the situation.

The problem for Blizzard is that a lot of PC gamers have fled that platform for mobile, so now money that used to go straight to its pockets is now shared. According to Gamastura, half of Hearthstone’s players have switched over to playing with their phones and tablets. Market research firm Newzoo predicts that mobile will be worth $30 billion in 2015, so it’s an important market for Blizzard to tap. The digital card game category is relatively new in mobile (it made $1.2 billion last year), and Blizzard’s Hearthstone dominates it.

Blizzard can offset that problem if the mobile versions bring in enough new users to cover that lost money from migrating PC players. There’s also the chance that new players might discover the game on mobile and later download and make in-app purchases on the PC version.