"files/" added before derivatives path

I'm using image module and watermark module.
Tonight I updated image module to 5.x 1.1, and tried the new rebuild thumbnails feature, rebuild process is ok, but watermark module reports that it cannot find the image file "files/files/images/xxx.jpg". then I checked the db, in files table, rebuilded derivatives paths begin with "files/images/..." but paths generated by older version begins with "images/....".
So watermark try to find the image with an extra "files" before its path, of course it won't find it...

is this a bug?