diff --git a/lib/mini_ca/certificate.rb b/lib/mini_ca/certificate.rb
index a851c58..1ad1989 100644
--- a/lib/mini_ca/certificate.rb
+++ b/lib/mini_ca/certificate.rb
@@ -26,7 +26,9 @@ def initialize(
       @counter = 0
 
       x509.version = 0x2
-      x509.serial = serial || 0
+      # https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.2
+      # must be unique positive integer
+      x509.serial = serial || OpenSSL::BN.rand(128, 0)
 
       x509.public_key = public_key
 
diff --git a/lib/mini_ca/version.rb b/lib/mini_ca/version.rb
index 3e6c417..8f14c68 100644
--- a/lib/mini_ca/version.rb
+++ b/lib/mini_ca/version.rb
@@ -1,3 +1,3 @@
 module MiniCa
-  VERSION = '1.2.1'.freeze
+  VERSION = '1.2.2'.freeze
 end