[imp] Strange error when using custom en_US.po

[Sean Page]

Hi group,
I have just enabled a custom en_US.po in imp 3.1.2 and horde 2.1.2 and
started recieving the following error:

Warning: htmlspecialchars() [function.htmlspecialchars]: charset `us-ascii'
not supported, assuming iso-8859-1 in
/usr/local/www/horde/imp/lib/MIME/Viewer/text.php on line 41

At first I was using charset= iso-8859-1 but now switched over to us-ascii
in the .po file but that doesn't seem to help. Any suggestions?

Sean.