"/xp -<amount>" doesnt not work, but "/xp -<amount>L" does

When I type the below command, it works fine, taking away 10 levels of experience from my player.
/xp -10L
But when I type THIS below command, it does not work, and it gives me an error message.
/xp -10
I REALLY want to be able to do this, as I can't summon XP orbs that give you a certain amount of levels. I want to take 10 XP (NOT levels) from the player, and then summon an XP orb with the "Value:10" tag on it.