Rewrite rule generating Internal Server Error

[eluser]johncook[/eluser]
The directory does exist and does match the regex (as far as I know). An example, the URL is:
http://cavaliers.criciwiki.com/images/header.jpg

I want it to rewrite to:
http://www.criciwiki.com/images/cavaliers/header.jpg

Trying both
RewriteRule ^images/(.*)$ images/cavaliers/$1 [L]
and
RewriteRule ^images/(.*)$ images/%1/$1 [L]
gives an Internal Server Error. It seems like trying to rewrite to a subdirectory in images is what causes the problem.

For the record, rewriting to just the images directory works okay (which confirms the regex matches):
RewriteRule ^images/(.*)$ images/$1 [L]