New to piwik, 1.6 upgrade Fatal Error

Hello all,

New to piwik. Installed via the web platform installer to IIS 7.5 runs fine. But once I attempt the 1.6 upgrade either manually or automatically. I recieve the following error afterwards:

Fatal error: Undefined class constant 'Piwik_Archive::INDEX_PAGE_ENTRY_NB_UNIQ_VISITORS' in C:\inetpub\analytics\core\PluginsManager.php on line 317

Any Ideas? Thanks in advance.

please try 1.7: 301 Moved Permanently