“Site blocked by robots.txt” — but there’s no robots.txt!

I’ve been trying to hook up my sitemap and make it visible to Google. Bu on the “add sitemap” page of Webmaster Tools I’m getting an error saying “URL restricted by robots.txt” — but I have no robots.txt! I made sure WordPress’ privacy setting is checked to allow search engines.

Apparently there’s a phantom robots.txt file somewhere, but I can’t figure out where. I’ve looked through all the directories on the server, checked all my pages for meta robots tags, etc. ???