swift2 - cannot invoke reduce with an argument list of type swift 2.0 -

when upgrade swift 1.2 swift 2.0 following error occurs

cannot invoke reduce argument list of type

here code

let escaped = reduce(string, "") { string, character in     string + (character == mark ? "\(mark)\(mark)" : "\(character)") 

can me how solve problem

reduce() method collections such arrays have call on list of characters can access using characters property of string, not on whole string itself:

let escaped = string.characters.reduce("") { string, character in     string + (character == mark ? "\(mark)\(mark)" : "\(character)") }