spf-a.prowly.com has no NS records.
NS records for this zone are defined at prowly.com.
NS records for this zone are defined at prowly.com.
Question and response
Question
dig spf-a.prowly.com. NS
Answer
The response has no answer records.
Authority
prowly.com. 900 SOA ns-19.awsdns-02.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0