When custom reject message is configured, non defined languages throw exception

I have this piece of authproc in one of my SPs:

    71 => [
      'class' => 'authorize:Authorize',
      'regex' => false,
      'GrouperGroups' => [
        'GN_Services:GN Project Participants',
      ],
      'reject_msg' => [
        'en' => 'This service is only available to GÉANT project participants.<br />' .
          'Please contact <a href="mailto:support@it.geant.org">support@it.geant.org</a> for support.',
        'nl' => 'Deze dienst is alleen beschikbaar voor deelnemers aan het GÉANT project.<br />' .
          'Neem contact op met <a href="mailto:support@it.geant.org">support@it.geant.org</a> voor ondersteuning.',
      ]
    ],

This works, but only for user that have their language set to en or nl. For users that have another language an exception is thrown:

SimpleSAML\Error\Exception: Error 2 - Illegal offset type in isset or empty at /ssp/wayf/modules/authorize/templates/authorize_403.php:18
Backtrace:
5 /ssp/wayf/www/_include.php:48 (SimpleSAML_error_handler)
4 /ssp/wayf/modules/authorize/templates/authorize_403.php:18 (require)
3 /ssp/wayf/lib/SimpleSAML/XHTML/Template.php:545 (SimpleSAML\XHTML\Template::show)
2 /ssp/wayf/modules/authorize/www/authorize_403.php:25 (require)
1 /ssp/wayf/lib/SimpleSAML/Module.php:254 (SimpleSAML\Module::process)
0 /ssp/wayf/www/module.php:10 (N/A)

simplesamlphp / simplesamlphp-module-authorize

When custom reject message is configured, non defined languages throw exception #2