u1.amazonaws.com has no NS records.
NS records for this zone are defined at amazonaws.com.
NS records for this zone are defined at amazonaws.com.
Question and response
Question
dig u1.amazonaws.com. NS
Answer
The response has no answer records.
Authority
amazonaws.com. 900 SOA dns-external-route53.us-east-1.amazonaws.com. hostmaster.amazon.com. 2017224478 180 60 604800 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0