Upgrading to 6: An HTTP error 0 occurred (says menu_router and menu_links tables don't exist)

I'm trying to upgrade a site to the latest Drupal 6 from Drupal 5. First, after uploading new files I was getting a 404 everywhere with an error like this. After following this advice and adding a language field in url_alias table manually the error was gone except it was warning of an unknown column "language" like that commenter said would be, but pages still were getting 404d.

However both prior and after this I am getting this when trying to run the upgrade at update.php:

An unrecoverable error has occurred. You can find the error message below. It is advised to copy it to the clipboard for reference.
Please continue to the error page
An HTTP error 0 occurred. http://www.freewaresupport.com/update.php?id=3&op=do

When I click to go to the error page I get a huuuuge red block of text, all user warnings complaining that tables menu_router and menu_links don't exist and showing the queries it apparently tried to apply to those. Here are the first couple of them:

# user warning: Table 'nuxorg.menu_router' doesn't exist query: DELETE FROM menu_router in /home/lighttpd/nuxified.org/http/includes/menu.inc on line 2326.
# user warning: Table 'nuxorg.menu_router' doesn't exist query: INSERT INTO menu_router (path, load_functions, to_arg_functions, access_callback, access_arguments, page_callback, page_arguments, fit, number_parts, tab_parent, tab_root, title, title_callback, title_arguments, type, block_callback, description, position, weight, file) VALUES ('node', '', '', 'user_access', 'a:1:{i:0;s:14:\"access content\";}', 'node_page_default', 'a:0:{}', 1, 1, '', 'node', 'Content', 't', '', 4, '', '', '', 0, '') in /home/lighttpd/nuxified.org/http/includes/menu.inc on line 2423.

Trouble is, these two tables are not supposed to exist in Drupal 5 from what I've seen. They should be created during upgrade, so what gives?