iMessage on desktop has a badge when iPhone does not - something to do with unknown senders

iMessage on my Desktop shows unread messages, even though there are none. It does not match my phone. If I right-click on the app icon, it shows an unread message from an unknown sender, but I have no way of viewing it and clearing it (even though it is marked "read" on my phone). The only way I can clear it is to delete the message from my phone. Is there a way to mark it as read?