A TimThumb error has occured

The following error(s) occured:

The remote file is not a valid image. Mimetype = 'text/html' File = /var/www/html/timthumb/cache/timthumbRPwJro

Query String : h=150&zc=1&a=c&src=http://www.elex.it/img/cultura_societa/CoperLibro1.jpg
TimThumb version : 2.8.13