domain/subdomain resolution

[Me]

hi all,
i have a situation that seems a little bit weired to me:

have 2 name servers: ns.mydomain.com and ns2.mydomain.com
they both host the dna records for the same network and subdomains.
today bind on ns.mydomain.com died. the ns2 is up. if i do :
nslookup mydomain.com
i get nonexistent domain
if i do:
nslookup mydomain.com ns2.mydomain.com
it gives me the right information..
the same with dig.
my question is how the nslookup doesn't find the domain but it finds the 
ns for that domain?
is there something i can do to fix that? how?
any help appreciated....
thank you