Substring matching for info index command

[Tom]

I've never used the builtin completion-styles, because I 
use other completion packages, but I thought I give it a try
and try substring matching for the info index command.

I added this advice to the command which wraps it in a
let and sets completion-styles to substring:

(defadvice Info-index (around my-Info-index activate)
  (let ((completion-styles '(substring)))
    ad-do-it))


However, it has no effect, it still uses the prefix
completion style. Why is that?