Unused comprehension throws "iterate(nothing)" method error

[FHell]

MWE:

using Zygote

function h(x)
    [2 for i in 1:2]
    sum(x)
end

gradient(h, zeros(2)) # ERROR: MethodError: no method matching iterate(::Nothing)

[mcabbott]

Complete error:

julia> gradient(h, zeros(2))
ERROR: MethodError: no method matching iterate(::Nothing)
Closest candidates are:
  iterate(::IRTools.Inner.IR) at /Users/me/.julia/packages/IRTools/GVPoj/src/ir/ir.jl:613
  iterate(::IRTools.Inner.IR, ::Any) at /Users/me/.julia/packages/IRTools/GVPoj/src/ir/ir.jl:613
  iterate(::IRTools.Inner.Pipe) at /Users/me/.julia/packages/IRTools/GVPoj/src/ir/ir.jl:875
  ...
Stacktrace:
 [1] _zip_iterate_some at ./iterators.jl:352 [inlined]
 [2] _zip_iterate_some at ./iterators.jl:354 [inlined]
 [3] _zip_iterate_all at ./iterators.jl:344 [inlined]
 [4] iterate at ./iterators.jl:334 [inlined]
 [5] iterate at ./generator.jl:44 [inlined]
 [6] collect(::Base.Generator{Base.Iterators.Zip{Tuple{Array{typeof(∂(#9)),1},Nothing}},Base.var"#3#4"{Zygote.var"#493#497"}}) at ./array.jl:686
 [7] map(::Function, ::Array{typeof(∂(#9)),1}, ::Nothing) at ./abstractarray.jl:2248
 [8] (::Zygote.var"#492#496"{Array{typeof(∂(#9)),1}})(::Nothing) at /Users/me/.julia/packages/Zygote/chgvX/src/lib/array.jl:187
 [9] (::Zygote.var"#520#521"{Zygote.var"#492#496"{Array{typeof(∂(#9)),1}}})(::Nothing) at /Users/me/.julia/packages/Zygote/chgvX/src/lib/array.jl:203
 [10] h at ./REPL[13]:2 [inlined]
 [11] (::typeof(∂(h)))(::Float64) at /Users/me/.julia/packages/Zygote/chgvX/src/compiler/interface2.jl:0
 [12] (::Zygote.var"#41#42"{typeof(∂(h))})(::Float64) at /Users/me/.julia/packages/Zygote/chgvX/src/compiler/interface.jl:45
 [13] gradient(::Function, ::Array{Float64,1}) at /Users/me/.julia/packages/Zygote/chgvX/src/compiler/interface.jl:54
 [14] top-level scope at REPL[14]:1

So it looks like the problem is in ∇map, around here:

Zygote.jl/src/lib/array.jl

Line 187 in 31a2b76

Δf_and_args_zipped = $mapfunc((f, δ) -> f(δ), _tryreverse($mapfunc, backs, Δ)...)