Validar campo no vacio

[IDRID]

Tengo un problema , no me detecta si el campo esta vacio.

Formulario :

<form name="Members" onsubmit="valido()">
<span class="LBP"><b>Enter your Code : </b></span><input type="text" name="codigo" value="Inserte su Codigo" onchange="valido()"><input type="button" value="Check In" name="Submit" onclick="valido()"></p>
</form>

JavaScript :

<script language='text/javascript'>
function valido() {
if (Members.codigo.value == &quot;&quot;) {
alert (&quot;Write your Code!&quot;);
return false; }
}
else {
validar();
}
}
</script>

Gracias!