Custom Script Not Running

I am running this custom script on Purchase Order Item Document.

Can you tell me why it is not running.

cur_frm.cscript.custom_validate = function(doc) {
if (doc.stock_uom!=doc.uom) {
cur_frm.set_value(“quantity”, doc.quantity/doc.conversion_factor);
}
}

try script on Purchase Order instead of Purchase Order Item

cur_frm.cscript.validate = function(doc) {
if (doc.stock_uom!=doc.uom) {
 cur_frm.set_value("quantity", doc.quantity/doc.conversion_factor);
}
}

Sorry, it didn’t work on Purchase Order also

Working script from gitter

frappe.ui.form.on('Purchase Order', 'validate', function(frm){
    if(frm.doc.items[0].prevdoc_docname){
    for (var idx in frm.doc.items){
        if (frm.doc.items[idx].stock_uom != frm.doc.items[idx].uom){
            frm.doc.items[idx].qty = frm.doc.items[idx].qty/frm.doc.items[idx].conversion_factor
        }
    }
    }
    });