dns1.shell.com has no NS records.
NS records for this zone are defined at shell.com.
NS records for this zone are defined at shell.com.
Question and response
Question
dig dns1.shell.com. NS
Answer
The response has no answer records.
Authority
shell.com. 600 SOA dns1.shell.com. dns.shell.com. 2830767 10800 1080 2419200 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0