His finger bent in the wrong direction when he got it caught between the mattress and the headboard.
It's a so-called drop finger. When I googled it, it said it's quite common in volleyball and handball, but in my defence, it actually also says it can happen when making the bed, says Mattias Karlsson.
Karlsson was due to play singles only at the European Championships in Ljubljana, which start on October 11th.
There wouldn't be a doubles event for me this year, which might make it feel a little easier.
