Duplicate entry when adding second user -- multi-site setup

After creating admin (user 1), I tried to create second user, got this error:

Duplicate entry '1' for key 1 query: INSERT INTO shared_users (name, mail, pass, init, status, uid, created) VALUES ('dave', 'user@whatever.com', '997cdfa03b55d9e7bee0fcbd69c1230a', 'user@whatever.com', 1, 1, 1165192401) in [...]/includes/database.mysql.inc on line 167.

I have a multi-site setup, sharing users/sessions tables like so:

$db_prefix = array(
  'default'        => 'js_',
  'authmap'        => 'shared_',
  'profile_fields' => 'shared_',
  'profile_values' => 'shared_',
  'role'           => 'shared_',
  'sequences'      => 'shared_',
  'sessions'       => 'shared_',
  'users'          => 'shared_',
  'users_roles'    => 'shared_',
);

The status message said success, but no record added to db. I then tried it again, and this time it worked.

I think this is related: reset password link (in the e-mail) takes me to a blank page (I turned on error messages), nothing in Drupal logs.

Is this due to multi-site? Anybody doing basic multi-site successfully w/ 5.0 beta2?

Thanks!