Enumerable#inject should handle empty enumerable; http://www.tbn.co.jp/blog/?p=813

mrblib/enum.rb

@@ -200,13 +200,18 @@ module Enumerable
   # ISO 15.3.2.2.11
   def inject(*args, &block)
     raise ArgumentError, "too many arguments" if args.size > 2
-    flag = true  # 1st element?
-    result = nil
+    if args.empty?
+      flag = true  # no initial argument
+      result = nil
+    else
+      flag = false
+      result = args[0]
+    end
     self.each{|val|
       if flag
-        # 1st element
-        result = (args.empty?)? val: block.call(args[0], val)
+        # push first element as initial
         flag = false
+        result = val
       else
         result = block.call(result, val)
       end