Error on calling the Xrm.Page.entity.save() method in the form onLoad event

Hi all,

here is a simple code that I don't understand why it doesn't work.

--------

function onLoad_account() {
    Xrm.Page.data.entity.attributes.get("fax").setValue("12345678");
    Xrm.Page.data.entity.save();
 }

-------

As you can see, in the OnLoad event I simply set the value of fax = "12345678" and then I save the record but the system give me the error "Object doesn't suport the property or method". Perhaps isn't possible save in the onload event?

Could you help me?

Thanks :)