rpmbuild error/question

I am using a vendor supplied script to build an rpm to be installed alongside some pre-built rpms provided by the vendor. The rpmbuild works just fine. On RHEL6, I can install this rpm no problem. On RHEL5, I get an error that the rpm I built is not signed. I can install it using

yum --nogpgcheck

--nogpgcheck

The vendor claims they cannot reproduce what I am seeing in their lab, and that it works for them as expected. They said to address with your OS vendor.

Is there something I'm missing here on the OS side that is causing this for me with RHEL5?