diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index b40d5c0359..dc33fffcb5 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -246,7 +246,7 @@ class IPAddr
   # Returns a string containing the IP address representation in
   # cidr notation
   def cidr
-    format("%s/%s", to_s, prefix)
+    "#{to_s}/#{prefix}"
   end
 
   # Returns a network byte ordered string form of the IP address.