[SOLVED] Failed to pull naming_series in customer to quotation print format

in Customer, there is field of naming_series.

the effect of series in customer like (Customer Series: CST16001):

So i using the code below to pull customer series no in quotation print format.
{% set u = frappe.get_doc("Customer",doc.customer) %} {{ u.naming_series or ''}}

but the effect is that, Customer Series no(CST16001) doesn’t display, only display series prefix (CST.YY…###),

Please help to check what’s problem and how to correct code.

Instead of this use {{ u.name }}

thanks very much