`treemap` is broken?

It looks like treemap is broken or at least maybe documentation example is needed?

using AbstractTrees
import AbstractTrees: children

struct Node
    v::Int
    c::Vector{Node}
end

children(n::Node) = n.c

tree = Node(1,
            [Node(2,
                  [Node(3, []),
                   Node(4, [])]),
             Node(5, [])])

With this setup, I wasn't able to run treemap in any configuration. FIrst of all, it accepts only PostOrderDFS types. Why can't I just pass tree and it internally call PostOrderDFS on it?

Secondly, call like this

treemap(x -> x.v, PostOrderDFS(tree))

return error

ERROR: MethodError: no method matching keys(::PostOrderDFS{Node})
Closest candidates are:
  keys(::Core.SimpleVector) at essentials.jl:605
  keys(::Cmd) at process.jl:639
  keys(::LibGit2.GitTree) at /home/skoffer/.julia/packages/Revise/XFtoQ/src/git.jl:52
  ...
Stacktrace:
 [1] pairs(::PostOrderDFS{Node}) at ./abstractdict.jl:134
 [2] treemap(::var"#19#20", ::PostOrderDFS{Node}) at /home/skoffer/.julia/packages/AbstractTrees/VQ0nX/src/iteration.jl:349

I can pirate it to ( I am completely at loss, what I am doing at this moment, actually)

Base.:pairs(x::PostOrderDFS) = enumerate(x)

This helps to go through this error, but it looks like function require more than one argument, so I change call to

treemap((i, x, c) -> x.v, PostOrderDFS(tree))

And then I get into error

ERROR: MethodError: no method matching copy!(::Array{Int64,1}, ::Int64, ::Array{Union{},1}, ::Int64, ::Int64)
Closest candidates are:
  copy!(::AbstractArray{T,1} where T, ::AbstractArray{T,1} where T) at abstractarray.jl:708
  copy!(::AbstractArray, ::AbstractArray) at abstractarray.jl:711
Stacktrace:
 [1] treemap(::var"#23#24", ::PostOrderDFS{Node}) at /home/skoffer/.julia/packages/AbstractTrees/VQ0nX/src/iteration.jl:369

Help shows that it is true, there is no copy! with 5 arguments in Base.

So, how this function is supposed to be used? Is it working at all?

Reproduce the Julia example and inspect treemap in iteration.jl, especially the code at the reported lines 349 and 369. Check how its traversal and callback arguments interact with PostOrderDFS and Base.copy!, then establish a supported call or document the required usage; done means the example runs without piracy or method errors.