URL Rewrite: Wrong redirect first character

Question

I got the following rule in my .htaccess (no other rules before or after this one):

RewriteRule ^archive/([^/\.]+)/?$ /archive.php?letter=$1 [L]

What I need this rule to do is the following:

mydomain/archive/letter rewrite to: mydomain/archive.php?letter=letter

It works for all letters of the alphabet, except for a.

The result is, while browsing:

mydomain/archive/b --> no redirect, shows page as it were archive.php?letter=b (correct)

mydomain/archive/a --> redirect to: mydomain// (incorrect)

I searched high and low, but I can't see what I'm doing wrong.

Answer 1

For some reason another rule after this one was interfering with the one in my question. I removed that one and it works now. So, the rule in the question is a working one :)