[jQuery] Validate on combobox

[ciupaz]

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