no NS but having A record

[Tech W.]

Hello,

For this domain, gdpu.cn, I tried to find its ns record:

dig gdpu.cn ns

with no results.

But I can dig its www record as below. 
why this happened? I can't understand entirely..
Thanks.

# dig www.gdpu.cn

; <<>> DiG 9.5.0-P2 <<>> www.gdpu.cn
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 59573
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;www.gdpu.cn.                   IN      A

;; ANSWER SECTION:
www.gdpu.cn.            2742    IN      A       219.136.229.45

;; AUTHORITY SECTION:
gdpu.cn.                13652   IN      NS      dns2.gdpu.cn.
gdpu.cn.                13652   IN      NS      dns1.gdpu.cn.