Backdrop error with static unfold

[Joaoloula]

The following works fine:

@gen (grad) function kernel(t::Int, (grad)(a::Real))
    z ~ normal(a, 1)
    return z
end

chain = Gen.Unfold(kernel)
T = 3
trace = Gen.simulate(chain, (T, 0.))

choice_gradients(trace, select(T => :z))

But if I change the annotation to (static, grad), all of a sudden gradients for the last timestep don't work anymore (strangely the gradients work fine for every timestep before that), and I get the following error:

MethodError: no method matching +(::Float64, ::Nothing)
Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...) at /Applications/Julia-1.7.app/Contents/Resources/julia/share/julia/base/operators.jl:655
  +(::Union{Float16, Float32, Float64}, ::BigFloat) at /Applications/Julia-1.7.app/Contents/Resources/julia/share/julia/base/mpfr.jl:413
  +(::AbstractFloat, ::Bool) at /Applications/Julia-1.7.app/Contents/Resources/julia/share/julia/base/bool.jl:172
  ...

Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/Gen/Dne3u/src/static_ir/backprop.jl:502 [inlined]
 [2] choice_gradients(trace::var"##StaticIRTrace_kernel#1797", selection::StaticSelection{(:z,), Tuple{AllSelection}}, retval_grad::Nothing)
   @ Main ~/.julia/packages/Gen/Dne3u/src/static_ir/backprop.jl:502
 [3] macro expansion
   @ ~/.julia/packages/Gen/Dne3u/src/static_ir/backprop.jl:396 [inlined]
 [4] choice_gradients(trace::var"##StaticIRTrace_kernel#1797", selection::DynamicSelection, retval_grad::Nothing)
   @ Main ~/.julia/packages/Gen/Dne3u/src/static_ir/backprop.jl:502
 [5] choice_gradients(trace::Gen.VectorTrace{Gen.UnfoldType, Float64, var"##StaticIRTrace_kernel#1797"}, selection::DynamicSelection, retval_grad::Nothing)
   @ Gen ~/.julia/packages/Gen/Dne3u/src/modeling_library/unfold/backprop.jl:39
 [6] choice_gradients(trace::Gen.VectorTrace{Gen.UnfoldType, Float64, var"##StaticIRTrace_kernel#1797"}, selection::DynamicSelection)
   @ Gen ~/.julia/packages/Gen/Dne3u/src/gen_fn_interface.jl:405
 [7] top-level scope
   @ In[482]:13
 [8] eval
   @ ./boot.jl:373 [inlined]
 [9] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
   @ Base ./loading.jl:1196

Not sure why this happens---any thoughts?