a cert write into database has no nickname

Dear Hubert,
                  I use pk12util to write a p12 cert into nssdb, and reply ok.
the p12 cert is include two certificates and private key, when i wrote, i use "pk12util -i   p12file   -n nickname   -d sql:./nssdb/"
it reply ok.
            when i use "certutil -L -d sql:./nssdb/" to list all cert, it display two certs one has default nickname, but another is "(NULL)",
and the "(NULL)" cert can not be delete by "certutil  -D -n (NULL)  -d sql:./nssdb/", could you tell me why? thank you!