I'm getting the following error logged when installing on zc 1.5.5f, running on PHP 5.6.30

PHP Warning: Illegal string offset 'SITEMAPXML_DIR_WS' in /Applications/MAMP/htdocs/tier1test/stOry-NTp-cOast/includes/modules/sitemapxml_install.php on line 34

Any ideas what is causing this?