[AUCTeX] I can not preview math formula .

[waterloo]

I use emacs 23.2.1 and auctex 11.85-1 in Debian 6 .
When I preview ' $\{a_n \}$ ' , this simple formula , I get the following errors :

> gs -dOutputFile\=\(_region_.prv/tmp5813419/pr1-1.png\) -q -dSAFER -dNOPAUSE -DNOPLATFONTS -dPrinted -dTextAlphaBits\=4 -dGraphicsAlphaBits\=4 -sDEVICE\=png16m -r155.52x155.384
> GS>{DELAYSAFER{.setsafe}if}stopped pop/.preview-BP currentpagedevice/BeginPage get dup null eq{pop{pop}bind}if def<</BeginPage{currentpagedevice/PageSize get dup 0 get 1 ne exch 1 get 1 ne or{.preview-BP 0.184314 0.309804 0.309804 setrgbcolor clippath fill 0.980392 0.921569 0.843137 setrgbcolor}{pop}ifelse}bind/PageSize[1 1]>>setpagedevice/preview-do{[count 3 roll save]3 1 roll dup length 0 eq{pop}{setpagedevice}{ifelse .runandhide}stopped{handleerror quit}if aload pop restore}bind def [(_region_.prv/tmp5813419/preview.ps)(r)file]aload exch dup 0 setfileposition 21394()/SubFileDecode filter cvx .runandhide aload pop dup dup 21394 setfileposition 137()/SubFileDecode filter cvx<<>>preview-do
> Error: /invalidfileaccess in --file--
> Operand stack:
>    --nostringval--   (_region_.prv/tmp5813419/preview.ps)   (r)
> Execution stack:
>    %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   %loop_continue   --nostringval--   --nostringval--   false   1   %stopped_push   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--
> Dictionary stack:
>    --dict:1160/1684(ro)(G)--   --dict:0/20(G)--   --dict:72/200(L)--
> Current allocation mode is local
> Last OS error: 2
> Current file position is 532
> GS<3>

What is the matter ?
Thanks first of all !