Can't send direct message to a follower

I'm not able to send a direct message to one of my followers. He is in my followers list, and I also follow him. When I visit his twitter page the "actions" section of the sidebar is completely missing, and when I try to send a message thru twhirl I get a dialog saying that the message failed and perhaps they aren't following me (but I already checked and they are).

Is this a temporary thing, or is there a setting that they may have enabled to prevent direct messages getting through?

Thanks.