BoundsError in NLP classify_linearity

[odow]

(jump) pkg> st
Status `/private/tmp/jump/Project.toml`
  [b6b21f68] Ipopt v0.6.3
  [4076af6c] JuMP v0.21.5

julia> using JuMP, Ipopt

julia> model = Model(Ipopt.Optimizer)
A JuMP Model
Feasibility problem with:
Variables: 0
Model mode: AUTOMATIC
CachingOptimizer state: EMPTY_OPTIMIZER
Solver name: Ipopt

julia> @variable(model, x)
x

julia> foo(x, y) = x + y
foo (generic function with 1 method)

julia> register(model, :foo, 2, foo, autodiff=true)

julia> ex = @NLexpression(model, foo(x^2, x))
"Reference to nonlinear expression #1"

julia> @NLobjective(model, Max, ex)

julia> optimize!(model)
ERROR: BoundsError: attempt to access 8-element Array{Symbol,1} at index [9]
Stacktrace:
 [1] getindex at ./array.jl:809 [inlined]
 [2] classify_linearity(::Array{JuMP._Derivatives.NodeData,1}, ::SparseArrays.SparseMatrixCSC{Bool,Int64}, ::Array{JuMP._Derivatives.Linearity,1}) at /Users/oscar/.julia/packages/JuMP/qhoVb/src/_Derivatives/linearity.jl:42
 [3] JuMP._SubexpressionStorage(::Array{JuMP._Derivatives.NodeData,1}, ::Array{Float64,1}, ::Int64, ::Array{JuMP._Derivatives.Linearity,1}, ::Dict{MathOptInterface.VariableIndex,Int64}) at /Users/oscar/.julia/packages/JuMP/qhoVb/src/nlp.jl:300
 [4] initialize(::NLPEvaluator, ::Array{Symbol,1}) at /Users/oscar/.julia/packages/JuMP/qhoVb/src/nlp.jl:363
 [5] optimize!(::Ipopt.Optimizer) at /Users/oscar/.julia/packages/Ipopt/bYzBL/src/MOI_wrapper.jl:1041
 [6] optimize!(::MathOptInterface.Bridges.LazyBridgeOptimizer{Ipopt.Optimizer}) at /Users/oscar/.julia/packages/MathOptInterface/k7UUH/src/Bridges/bridge_optimizer.jl:264
 [7] optimize!(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}) at /Users/oscar/.julia/packages/MathOptInterface/k7UUH/src/Utilities/cachingoptimizer.jl:215
 [8] optimize!(::Model, ::Nothing; bridge_constraints::Bool, ignore_optimize_hook::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/oscar/.julia/packages/JuMP/qhoVb/src/optimizer_interface.jl:130
 [9] optimize! at /Users/oscar/.julia/packages/JuMP/qhoVb/src/optimizer_interface.jl:106 [inlined] (repeats 2 times)
 [10] top-level scope at REPL[11]:1

Reported by @MFairley