Exec with tar commande to make archive files before upgrade need path

During tests about core::update we got the following message : error during compilation: Validation of Exec[www.foo.org > Archive files before upgrade] failed: 'tar -cvf /var/fooarchives/www.foo.org_2018-07-31.tar.gz .' is not qualified and no path was specified. Please qualify the command or specify a path.