WordPress MU

When uploading an image I get:

Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /home/mydir/public_html/wp-admin/includes/media.php on line 736

A quick google for the error shows this recent thread over on the wp.org site:

http://wordpress.org/support/topic/224164

Have to admit that I'm scratching my head on that one.

I'm using WPMU 2.7

Following the fix described for WP 2.7 @
http://wordpress.org/support/topic/224164

... I opened:

/wp-admin/includes/media.php

And changed this:

( !array_key_exists($checked, $alignments) )

To this:

( !array_key_exists('$checked', $alignments) )

i.e. added ' either side of $checked

The fix described at the above link specifies the location of the file incorrectly, but a later poster on that thread mentions the correction.