each_with_index yields invalid values when custom Enumerable class calls Proc#call

On 1.9 compatibility mode, each_with_index yields invalid values
when a custom Enumerable class calls Proc#call for the given block in its `each` implementation.
Diff for the test case included. (Test passes on MRI.)

###

class CustomEnumerableWithProcCall
include Enumerable
def each &block
block.call 'hello'
end
end

CustomEnumerableWithProcCall.new.each_with_index do |e, i|
p [e, i]
end

###

["hello", 0] expected. but [["hello", nil], 0]