Custom adjoint for getindex in Zygote

I am new to Zygote and have a question about defining my own adjoint involving getindex with custom indexing.

I’ve defined a type

import Base: getindex

struct ProbTensor{N}

function getindex(e::ProbTensor, idx...)
  sum(view(e.elements, (i==0 ? Colon() : (i+3)÷2 for i in idx)...))

to represent a probability distribution over N Boolean values, I’m storing false values at position 1 and true values at position 2 and I represent these values in other parts of the code as false=-1, true=1.

I’ve overloaded getindex to convert the -1/+1 indices into the array positions 1/2 and I’ve overloaded an index of 0 to correspond to marginalizing over the true and false cases. E.g. (p::ProbTensor{3})[0,0,0] should sum over all elements and return 1.0.

Now, I’m trying to define an adjoint for derivatives of the output of getindex with respect to the stored probabilities. My first attempt at the gradient of the normalization

using Zygote: gradient, @adjoint

probs = rand(2,2);  probs = probs / sum(probs)

gradient(p->p[0,0], ones(2,2))

fails by complaining about accessing element [0,0].

So, I tried to define my own adjoint

covers(idx1::Int, idx2::Int) = idx1 == 0 || (idx1==-1 && idx2==1) || (idx1==1 && idx2==2)
covers(idxs1, idxs2::Tuple) = all(covers.(idxs1,idxs2))
covers(idxs1, idxs2::CartesianIndex) = covers(idxs1, Tuple(idxs2))

@adjoint getindex(e::ProbTensor, idx...) =
  (getindex(p, idx...), dp -> (begin
                                              arr = zeros(eltype(dp), size(dp))
                                              for i in CartesianIndices(dp)
                                                covers(idx,i) && (arr[i] = dp[i])

(covers checks for the non-zero derivative elements)

But I still get the same error. Can someone please help set me on the correct path?