Use NilClass when the parameter type varies and is optional e.g.

def b(o=nil)
puts o.class
end

b 'a'
#=> String

b
#=> NilClass

Resources:
- Class: NilClass [ruby-doc.org]

Read more: http://feeds.dzone.com/~r/dzone/snippets/~3/O1QsQfQ_qa8/11635