I am defining and ODESystem using ModellingToolkit and I am running into a problem in the definition of the equations.

The code below is the function to define the ODESystem

```
function connectFFFB_Grad(plant, ref, pid, mlp,lookahead_times,N,plantMatrices;name=:FF_FB_G)
@parameters t
dt = Differential(t)
@variables intError(t)
# @variables dydw[1:N](t) #grad output wrt weights
@variables dx1dw[1:N](t) #grad 4th plant state wrt weights
@variables dx2dw[1:N](t) #grad 4th plant state wrt weights
@variables dx3dw[1:N](t) #grad 4th plant state wrt weights
@variables dx4dw[1:N](t) #grad 4th plant state wrt weights
A,B,C,D = plantMatrices
# @variables e(t)
eqs = vcat(
get_input(pid,1) ~ get_output(ref, 1) - get_output(plant,1),
get_output(pid,1) ~ get_input(plant, 1),
get_output.((ref,), 2:length(lookahead_times)+1) .~ get_inputs(mlp,length(lookahead_times)),
get_output(mlp,1) ~ get_input(plant,2),
dt(intError) ~ abs2(get_output(ref,1) - get_output(plant,1)),
dt.(dx1dw) .~ A[1,:]*vcat(dx1dw,dx2dw,dx3dw,dx4dw),
dt.(dx2dw) .~ A[2,:]*vcat(dx1dw,dx2dw,dx3dw,dx4dw),
dt.(dx3dw) .~ A[3,:]*vcat(dx1dw,dx2dw,dx3dw,dx4dw),
dt.(dx4dw) .~ A[4,:]*vcat(dx1dw,dx2dw,dx3dw,dx4dw),
# dt.(dydw) .~ C*[A*vcat(dx1dw,dx2dw,dx3dw,dx4dw).+B[:,2]*get_hiddens(mlp,N)],
# # e ~ abs2(get_output(ref,1) - get_output(plant,1)),
)
return ODESystem(eqs; systems=[plant,ref,pid,mlp],name=name)
end
```

I am getting the following error

```
ERROR: MethodError: no method matching *(::Vector{Int64}, ::Vector{Num})
Closest candidates are:
*(::Any, ::Any, ::Any, ::Any...) at operators.jl:560
*(::StridedMatrix{T}, ::StridedVector{S}) where {T<:Union{Float32, Float64, ComplexF32, ComplexF64}, S<:Real} at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/matmul.jl:44
*(::StridedVecOrMat{T} where T, ::LinearAlgebra.LQPackedQ) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/lq.jl:278
...
Stacktrace:
[1] connectFFFB_Grad(plant::ODESystem, ref::ODESystem, pid::ODESystem, mlp::ODESystem, lookahead_times::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}}, N::Int64, plantMatrices::NTuple{4, Matrix{Int64}}; name::Symbol)
```

I know that the error is in the te last four equations as I had previously function defining the same system excluding those four variables and it worked well.

It seems that the multiplication between the matrix column and the vector with the variables is not defined but I don’t understand why. Especially as I have another ODESystem defined by the following function that doesn’t pose any problems.

```
function linear_plant(A,B,C,D; name=:lp)
num_inputs = size(B,2)
num_states = size(A,1)
num_outputs = size(C,1)
@parameters t
dt = Differential(t)
@variables x[1:num_states](t)
@variables u[1:num_inputs](t)
@variables o[1:num_outputs](t)
eqs = vcat(
dt.(x) .~ A*x .+ B*u,
o .~ C*x .+ D*u
)
return ODESystem(eqs;name=name)
end
```

A is a 4x4 matrix defined as follows

```
A = [0 1 0 0; -2 -2 1 0; 0 0 0 1; 0 0 -2 -2];
```

I would appreciate any help understanding this error.