A TimThumb error has occured

The following error(s) occured:

The remote file is not a valid image. Mimetype = ''/var/www/vhosts/desterkerij.nl/httpdocs/includes/cache/timthumbKz9Qpp

Query String : w=1024&h=768&src=http://www.bandwerkplus.nl/klanten/www.desterkerij.nl/uploads/producten/groot/99.png
TimThumb version : 2.8.14