I put 

    # po plugin
    po_master_language => 'en|English',

    po_slave_languages => [ 'zh|Chinese']


in page.setup. And did 
  
    $ikiwiki --setup ./page.setup

but get errors:

    Can't use string ("en|English") as a HASH ref while "strict refs" in use at /usr/share/perl5/IkiWiki/Plugin/po.pm line 162.

Line 162 of the file reads

   delete $config{po_slave_languages}{$config{po_master_language}{code}};;

What's wrong? How to fix it?