Default arguments in Function attribute of type

This works:

type Dude
    say_something::Function
    # constructor
    function Dude()
        dude = new()

        dude.say_something = function(what::AbstractString)
            print(what)
        end
        return dude
    end
end

dude = Dude()
dude.say_something("hi")

But I want a default argument for what. However, this code:

type Dude
    say_something::Function
    # constructor
    function Dude()
        dude = new()

        dude.say_something = function(what::AbstractString="hi")
            print(what)
        end
        return dude
    end
end

dude = Dude()
dude.say_something("hello")

yields an error:

ERROR: LoadError: syntax: expected "(" in function definition in include at ./boot.jl:261 in include_from_node1 at ./loading.jl:304 in process_options at ./client.jl:280 in _start at ./client.jl:378

Are default arguments or keyword arguments not allowed in a Function attribute of a type?