Multiple column sort does not work

I had some trouble with a 3 columns sort (Category / Last Name / First Name) and eventually found a problem with the indices:

--- webform_report.inc.orig     2010-10-25 16:29:25.000000000 +0200
+++ webform_report.inc  2010-10-25 16:45:01.000000000 +0200
@@ -701,7 +701,7 @@
         $code = '';
         for ($i=0; $i<count($a); $i++) {
           if ($i < count($a) - 1) {
-            $code .= 'if (($tmp = ' . $a[0] . ') == 0) { ';
+            $code .= 'if (($tmp = ' . $a[$i] . ') == 0) { ';
           }
           else {
             $code .= '$tmp = ' . $a[$i] . '; ';

Seems to work now...