Writing with NPZ

I am trying to write a dictionary element that is a list of lists of Float64. Here is my MWE:

import NPZ
d = Dict{String, Vector{Vector{Float64}}}("tdnn" => [])
push!(d["tdnn"],  [1.,2.,3.])
push!(d["tdnn"],  [4.,2.,3.])
NPZ.npzwrite("gordon.npz", d)
NPZ.npzwrite("gordon.npz", d["tdnn"])

I have create a dictionary d with two Float64 lists of 3 elements each. Here is the error I get when applying npzwrite. Both statements generate the same error:

ERROR: LoadError: ArgumentError: cannot reinterpret `Vector{Float64}` as `UInt8`, type `Vector{Float64}` is not a bits type
 [1] throwbits at ./reinterpretarray.jl:16
 [2] reinterpret at ./reinterpretarray.jl:41
 [3] npzwritearray at /Users/erlebach/.julia/packages/NPZ/UCofn/src/NPZ.jl:360
 [4] npzwrite at /Users/erlebach/.julia/packages/NPZ/UCofn/src/NPZ.jl:437
 [5] top-level scope at /Users/erlebach/src/2022/rude/giesekus/GE_rude.jl/tst.jl:5
 [6] eval at ./boot.jl:368

I do not understand why the need for the reinterpretation. What bit type condition is the error referring to? Must the list of lists be converted to an array before npzwrite can work properly?

I figured it out. npzwrite can only write arrays of Numbers, which excludes strings and Vectors of Vectors. I used reduce(hcat, list) to convert my list of lists into a single array.