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Error in named-checkzone: DNS zone error:( out of zone) has no addresses records (A or AAAA)

I have the same issue as in Error in named-checkzone. I have cross-checked my zone file and they are properly formatted FQDN.

I checked the zone file using :

:/etc/bind$ sudo named-checkzone linuxconfig.org zones/master/db.linuxconfig.org 
zone linuxconfig.org/IN: sample.linuxconfig.org/NS 'ns1.sample.linuxconfig.org' (out of zone) has no addresses records (A or AAAA)
zone linuxconfig.org/IN: loaded serial 1
OK

The file: db.linuxconfig.org. contains the following:

;
; BIND data file for linuxconfig.org
;
$TTL    3h
@       IN      SOA     ns1.linuxconfig.org. admin.linuxconfig.org. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 day
;
@       IN      NS      ns1.linuxconfig.org.    

linuxconfig.org.            IN      MX      10      mail.linuxconfig.org.
linuxconfig.org.            IN      A       192.168.0.19
ns1                         IN      A       92.168.0.10
www                         IN      CNAME   @
mail                        IN      A       12.168.0.10
ftp                         IN      CNAME   linuxconfig.org.
sample                      IN      NS      ns1.sample.linuxconfig.org.
ns1.sample.linuxconfig.org. IN      A       1.1.1.1

If I removed the ns1.sample.linuxconfig.org. record then BIND will complain .. has no REQUIRED GLUE address records.

Why is the named-checkzone complaining when BIND can successfully accept the file and return the records when queried?