Setting SelectedIndex Based On Value?

I have the following snippet I'm trying to make work in Firefox:

Code:

for(var j=0; j<sBox.options.length; j++){
if(sBox.options[j].value == 68)
sBox.selectedIndex = j;
}

No errors are thrown and the correct index is not selected.

However, when I add the following line to the snippet, it magically works fine:

Code:

alert(sBox.options.length);
for(var j=0; j<sBox.options.length; j++){
if(sBox.options[j].value == 68)
sBox.selectedIndex = j;
}

Instead of using alert, I've tried simply assigning sBox.options.length to throwaway variable, but I end up with the same results as the first snippet.