How to join the content of an array in an EPP template ?

Hello, according to the following documentation page : https://docs.puppet.com/puppet/latest/reference/lang_template_epp.html I should be able to use the join function to merge the content of an array. I tried a lot of things without success :

$ cat test3.epp<%- | Array $test_array_join
| -%><%= join($test_array_join,'---') %>
$ puppet epp render test3.epp --values "{ 'test_array_join' => [ 'a', 'b', 'c' ] }"
Error: Evaluation Error: Unknown function: 'join'. at /home/git/features/dmzin_news_ftp/puppet/modules/proftpd/templates/test3.epp:5:5
Error: error while rendering epp
Error: Try 'puppet help epp render' for usage
$

I also try to isolate the code from the documentation example, same results :

$ cat test4.epp<%- |
      Array   $keys_trusted
| -%><% unless $keys_trusted =~ Array[Data,0,0] { -%>
trustedkey <%= $keys_trusted.join(' ') %><% } -%>
$ puppet epp render test4.epp --values "{ 'keys_trusted' => [ 'a', 'b', 'c' ] }"
Error: Evaluation Error: Unknown function: 'join'. at /home/git/features/dmzin_news_ftp/puppet/modules/proftpd/templates/test4.epp:5:34
Error: error while rendering epp
Error: Try 'puppet help epp render' for usage
$

From my understanding, I can use all Puppet functions inside EPP, but join() is a function of the puppet/stdlib. Please help me, I need some features introduced by EPP template, but if I can't join array, it will be complicated :(

$ puppet --version
4.3.1

Thanks in advance Best regards Denis Sacchet