Examples of mortar() for nd arrays?

The docstring for mortar() shows

mortar(blocks::AbstractArray{R, N}, sizes_1, sizes_2, ..., sizes_N)

But I can't figure out how to use this. Here's what I expected to work:

julia> arr = [[rand(2,3,4)] [rand(2,3,4)]
              [rand(2,3,4)] [rand(2,3,4)]];

julia> typeof(arr)
Matrix{Array{Float64, 3}} (alias for Array{Array{Float64, 3}, 2})

julia> mortar(arr, [2,2], [3,3], ones(Int, 4))
ERROR: MethodError: no method matching mortar(::Matrix{Array{Float64, 3}}, ::Vector{Int64}, ::Vector{Int64}, ::Vector{Int64})

I was expecting this to produce the equivalent of

julia> BlockedArray(rand(4,6,4), [2,2], [3,3], ones(Int, 4))
2×2×4-blocked 4×6×4 BlockedArray{Float64, 3}:
# ...

Start with the mortar() docstring and reproduce the reported Julia REPL call using the shown arr and block sizes. Compare it with the BlockedArray(rand(4,6,4), ...) example; done means documenting the correct nd-array usage or clearly stating the supported limitation.