RewriteCond giving 500 error and not 403

I've noticed in my logs that if I ban a useragent like below:

RewriteCond %{HTTP_USER_AGENT} ^libwww-perl [OR]

It returns a 500 error rather than a 403

Should this cause me any alarm or do i need to change something. It works when I use this:

deny from xxx.xxx.xxx.xxx

I have this at the start of the section for banned ip's and user agents

<Files 403.shtml>
order allow,deny
allow from all
</Files>

My last line is this:

RewriteCond %{HTTP_USER_AGENT} ^psycheclone
RewriteRule ^.* - [F]

Any tips or help on this would be appreciated.

Thanks Ian

 

Try the E flag. So new code is:

RewriteCond %{HTTP_USER_AGENT} ^libwww-perl [OR, E=403]