Replace Up Arrow with Shift+Tab

[Andrus]

I need to move to previous input field when Up arrow is pressed
in checkbox (IE 5.5+)

I try to replace Up Arrow keypress with Shift+Tab keypress in onkeydown
event.

Unfortunately, this does not work:

error: member not found

occurs at line
event.shiftKey=true;

How to move to previous input field if up arrow is pressed ?

Code to reproduce the problem:

<input type=checkbox
onkeydown="
if(!event.ctrlKey && event.keyCode==38) {
event.shiftKey=true;
// next line causes error: Member not found
event.keyCode=9;
return; }">