When attempting to upgrade from 1.9.8 the following error occurred:

Fatal error: Call to undefined function redirect_if_major_upgrade_required() in [path to moodle root]/index.php on line 35

Assuming this to be a check for versions lower than 1.9, I commented the line out and then got the following errors:

Notice: Undefined variable: PAGE in [path to moodle root]/index.php on line 43

Fatal error: Call to a member function set_url() on a non-object in [path to moodle root]/index.php on line 43