[may be OT] What is the meaning of this error from dnsreport ?

[Netfortius]

As a preliminary step for some work I need to do on a client's site, on some 
firewall networking and rules setup for the DMZ (including TCP 53 for DNS 
servers communications restrictions), I ran a dnsreport on my client's domain 
name:

http://dnsreport.com/tools/dnsreport.ch?domain=<my-cllent's-domain>.com

and I got this error:

FAIL	Missing nameservers 2	ERROR: One or more of the nameservers listed at the 
parent servers are not listed as NS records at your nameservers. The problem 
NS records are: <ns-of-ISP-hosting-secondary-server>

My client is hosting his DNS primary server in house, with the secondary setup 
at the ISP, and the dnsreport did not come up with any other errors, so:
- the parents server report them both as authoritative
- a change I have forced - out of curiosity - on the serial number on my 
client's hosted name server showed up at the secondary, so they are "aware" 
of each other.

Having described the above - what could be the cause of the problem reported 
by dnsreport.com (i.e. is that a mis-configuration at my client's site, or at 
the ISP's name server)?

TIA,
Stefan