Serial number should be a positive unique number.
The ruby doc for the X509 cert stated it should be a 2 digit byte range - i updated our code accordingly.
Ref - http://www.ruby-doc.org/stdlib-1.9.3/libdoc/openssl/rdoc/OpenSSL/X509/Certificate.html -
"Secure choices are integers in the two-digit byte range and ideally not sequential but secure random numbers"