BUG #2660: Cannot install ISN extension when charset=UTF88

The following bug has been logged online:

Bug reference: 2660
Logged by: Tim N. van der Leeuw
Email address: tnleeuw@gmail.com
PostgreSQL version: 8.2beta1
Operating system: WinXP Pro, SP2
Description: Cannot install ISN extension when charset=UTF88
Details:

When installing PostgreSQL 8.2beta1, I selected charset 'UTF-8'.
When selecting extra features to install (from the installer), I clicked on
'ISN'.

The installer then gave me an error-message with the text:

a database command error occurred whilst executing 'C:\Program
Files\PostgreSQL\8.2beta1\share\contrib\isn.sql':

ERROR: Invalid byte sequence for encoding "UTF8": 0xe1620

(NB: The error window didn't allow copy-pasting of the exact text, so I
typed over what I thought was the most relevant part of it. Unfortunately I
cannot instantly attach the screenshot when entering the bug)