Select che abilita altra select

[Giubass]

Ho provato così ma non funge

function Attiva(){
var sel = document.getElementById('Categoria'); 
if(sel.options[sel.selectedIndex].value != '-1'){
document.getElementById('Sub_Categoria').disabled=false;
}else{
document.getElementById('Sub_Categoria').disabled=true;
}

<select name="Categoria" size="1" id="Categoria" onchange="Attiva();">
<option value="-1" selected="selected">- Seleziona -</option>
<option value="1">Prova 1</option>
<option value="2">Prova 2</option>
</select>

<select name="Sub_Categoria" size="1" id="Sub_Categoria" disabled>
<option value="-1" selected="selected">- Seleziona -</option>
<option value="1">Prova 1</option>
<option value="2">Prova 2</option>
</select>