Could not find maxima-index.lisp



Hi everyone, I got an error when I to use the built-in help of maxima:

(%i1) ?? diff

Maxima encountered a Lisp error:

 Couldn't load #P"/usr/share/info/./maxima-index.lisp": file does not
exist.

Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.
; 
; compilation unit aborted
;   caught 1 fatal ERROR condition
(%i2) 

Am I wrong in something? Above there are my system informations

(%i2) build_info();

Maxima version: 5.23.2
Maxima build date: 14:11 1/18/2011
Host type: x86_64-unknown-linux-gnu
Lisp implementation type: SBCL
Lisp implementation version: 1.0.40.0.debian

(%o2) 

I'm using Ubuntu.

MR