[jQuery] Validate on combobox

Hi all,
I need to validate a combobox if the user doesn't make a choice.
My combo is:
<select id="utenza" name="utenza">
<option value="-1">null</option>
<option value="1">utenza1</option>
<option value="2">utenza2</option>
</select>
and I'm trying this code:
rules: {
utenza: {
required: "#utenza[selectedValue=-1]"
}
},
but it doesn't work.
How could fix it?
Thanks in advance.
Luigi