Adding a record for a.b.example.com makes b.example.com NXDOMAIN when b was served only by a *.example.com wildcard
NXDOMAIN
Before creating any record under a name that currently resolves only via a wildcard, first create an explicit record for the parent:
vercel dns add example.com customer A 203.0.113.10 # explicit parent first
dig +short customer.example.com @<authoritative-ns> # verify
vercel dns add example.com app.customer A 198.51.100.7 # then the child
If already broken, adding the explicit parent record restores it (clients may still see the cached NXDOMAIN until the SOA minimum/negative TTL, ~600 s here, expires).
Symptom
A zone serves many hosts through one wildcard record (* A 203.0.113.10). After adding a record for a deeper name such as app.customer.example.com, the parent customer.example.com — which had been working through the wildcard — suddenly returns NXDOMAIN / "No answer". The web server is healthy; only DNS broke. The outage lasts at least the SOA negative TTL even after the fix.
Cause
RFC 4592: once app.customer exists, customer becomes an empty non-terminal — a name that exists in the zone only because it has children. Wildcards never synthesize answers for names that exist, so *.example.com stops matching customer.example.com. This is standard DNS behaviour, not a provider bug (observed on Vercel DNS).
Verify
dig customer.example.com @ns1.<provider> returns the A record; public resolvers return it after the negative TTL expires.
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