DNS Reverse Lookup failing

Hi. I’m in the process of transfering the host of my DNS domain to a locally managed server. I have the server configured with foward and reverse lookup zones in Windows 2000. However when I do a nslookup on the Primary DNS servers IP address I get the following…
c:> nslookup -q =ptr 53.21.100.1
Server: name1.mydomain.com
Address: 53.21.100.1
*** name1.mydomain.com cant find =ptr Server failed

Why can’t I do a reverse lookup on this address ?

rbowe@familyguardian.com