gistfile1.pp

$service_addr6 = hiera('network::service6', undef)

gistfile2.erb

<%- if @service_addr6 then -%>
    <%- @service_addr6.each do |addr6| -%>
    ip-address: <%= addr6.split("/")[0] %>
    <%- end -%>
<%- end -%>

gistfile3.txt

  Filepath: /etc/puppetlabs/puppet/environment/network_test/modules/nsd/templates/nsd.conf.erb
  Line: 17
  Detail: undefined method `each' for "":String