Why insert a record before the origin record(begins with a blank) will make it invalid?

Issue

• When inserting a record before the origin record(begins with a blank) will make it invalid.
  For example

# Before adding the record.
[root@localhost named]# cat domaintest.com.zone 
$ORIGIN domaintest.com.
<..snip..>
 NS @
 A 192.168.30.101
 IN MX 10 mail.google.com. ; this is an origin mx record for domaintest.com
[root@localhost named]# dig @localhost mx domaintest.com +short
10 mail.google.com.


# After adding the record.
[root@localhost named]# vim domaintest.com.zone
[root@localhost named]# rndc reload
server reload successful
[root@localhost named]# cat domaintest.com.zone
$ORIGIN quanmytest.com.
<..snip..>
   NS @
   A 192.168.30.101
aa IN A 192.168.30.102 ; insert a record before origin mx record
   IN MX 10 mail.google.com.

[root@localhost named]# dig @localhost mx domaintest.com +short
[root@localhost named]#  <<<-- The origin record is invalid.