undefined method `sort_by' for nil:NilClass

[xprntl]

err: Could not retrieve catalog from remote server: Error 400 on SERVER: Failed to parse template fluentd/source.erb:
Filepath: /etc/puppet/modules/fluentd/templates/source.erb
Line: 2
Detail: undefined method `sort_by' for nil:NilClass
at /etc/puppet/modules/fluentd/manifests/configfile.pp:4 on node app01