Add Primary Contact to Payment Entry using Custom Script

1

I am trying to call primary contact using custom script if party is customer but it’s not working. Here’s my sctipt;

frappe.ui.form.on('Payment Entry', 'onload', function(frm){
    if(frm.doc.__islocal){
		if(frm.doc.party_type == "Customer") {	
		   frm.add_fetch('customer', 'customer_primary_contact');
		}
	}
});

Is there something I’m missing?