Uncertain what warning means

[TorkelE]

The following code:

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
@variables X(t) Y(t)
@parameters d 
using ModelingToolkitNeuralNets, Lux
nn_arch = Lux.Chain(
    Lux.Dense(1 => 3, Lux.softplus, use_bias = false),
    Lux.Dense(3 => 3, Lux.softplus, use_bias = false),
    Lux.Dense(3 => 1, Lux.softplus, use_bias = false)
)
nn, θ = SymbolicNeuralNetwork(; nn_p_name = :θ, chain = nn_arch, n_input = 1, n_output = 1)
eqs_ude = [
    D(X) ~ nn([Y], θ)[1] - d*X
    D(Y) ~ X - d*Y
]
@mtkcompile xy_model_ude = System(eqs_ude, t)

generates a warning:

┌ Warning: Internal error: Variable Y(t) was marked as being in Differential(t)(X(t)) ~ (NN(Num[Y(t)], θ))[1] - d*X(t), but was actually zero
└ @ ModelingToolkit.StructuralTransformations ~/.julia/packages/ModelingToolkit/b28X4/src/structural_transformation/utils.jl:292

However, I am uncertain exactly what it means, if it is a worry, and what to do about it.

[SebastianM-C]

This is a known issue with MTK and vector variables, it should be safe to ignore. We should document this and link to the MTK issue