julia: problematic convesion to Vector{Tuple}

conversion_trouble.jl

typealias MyTuple (ASCIIString, BitsKind)

type MyType
    some_field::Int
    function MyType(vec::Vector{MyTuple})
        println("INNER")
        new(0)
    end
end

function MyType(vec)
    println("OUTER")
    println("  eltype pre=$(eltype(vec))")
    c_vec = convert(Vector{MyTuple}, vec) ## this conversion doesn't work!
    println("  eltype post=$(eltype(c_vec))")
    MyType(c_vec)
end

some_data   = [("a", Int32), ("b", Int32)]
some_c_data = convert(Vector{MyTuple}, some_data) ## conversion works here

ex_c_data = MyType(some_c_data) ## <- this works
#ex_data   = MyType(some_data)  ## <- this keeps calling the
                                ##    outer constructor