Incorrect URI links for commenters

When someone leaves a comment and puts their URI in the corresponding field, my site shows that URI appended to the end of the entry URI by their posted comment (e.g., (http://myURI/postdate/postname/theirURI).

I’m using the custom URI structure and WP 1.5. I get the same results with the two packaged themes (and an additional two other ones I tried). How can I fix this?