spf.digitalocean.com has no NS records.
NS records for this zone are defined at digitalocean.com.
NS records for this zone are defined at digitalocean.com.
Question and response
Question
dig spf.digitalocean.com. NS
Answer
The response has no answer records.
Authority
digitalocean.com. 1800 SOA kim.ns.cloudflare.com. dns.cloudflare.com. 2397267224 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0