WP template editor unable to access 'index.php' (by default)

[mark]

When clicking on the 'templates' link (templates.php) in the admin panel, it gives the following error:

Editing index.php

Oops, no such file exists! Double check the name and try again, merci.

The same error is re-produced when clicking the 'Main Index' link listed under 'Common files: (click to edit)'. When typing in './index.php', however, one is able to access 'index.php'. All other links work in the template editor.