Set Dom Element Class (if Element Was Dynamically Created)

hi everybody, didn't find this using the search :( this is my problem:
i create a dom element dynamically (<span>) and want to assign a class
attribute to it such that it has some css style, this works in ie, but
not in firefox :( here's the simple code:

<div id="somediv"></div>

var div = window.document.getElementById("somediv");
var span = window.document.createElement("span");
span.innerHTML = "span";
span.attributes.getNamedItem("class").nodeValue = "span_class";
div.appendChild(span);

so this works well in ie6, but firefox gives this error:
Error: span.attributes.getNamedItem("class") has no properties

if i run this code on a statically defined span (in the html file),
where a class attribute already is set:

<div id="somediv">
<span id="spanspan" class="">
</span>
</div>

changing the class to "span_class" works both in ie and firefox.

how can i fix my above code such that it works in both browsers? it
seems that on newly created dom elements the class attribute isn't even
there (to change it), how can i create it, and then change it?