I’m trying to follow the example for solving the heat equation PDE from the DiffEqOperators page, but I am running into an error in the discretization step. The code is

```
using OrdinaryDiffEq, ModelingToolkit, DiffEqOperators
# Parameters, variables, and derivatives
@parameters t x
@variables u(..)
# Warning: `@derivatives D'''~x` is deprecated. Use `Differential(x)^3` instead.
#@derivatives Dt'~t
#@derivatives Dxx''~x
Dt = Differential(t)
Dxx = Differential(x)^2
# 1D PDE and boundary conditions
eq = Dt(u(t,x)) ~ Dxx(u(t,x))
bcs = [u(0,x) ~ cos(x),
u(t,0) ~ exp(-t),
u(t,Float64(pi)) ~ -exp(-t)]
# Space and time domains
domains = [t ∈ IntervalDomain(0.0, 1.0),
x ∈ IntervalDomain(0.0, Float64(pi))]
# PDE system
pdesys = PDESystem(eq,bcs,domains,[t,x],[u])
# Method of lines discretization
dx = 0.1
order = 2
discretization = MOLFiniteDifference(dx,order)
# Convert the PDE problem into an ODE problem
prob = discretize(pdesys,discretization) # <--- Error occurs here
# Solve ODE problem
sol = solve(prob,Tsit5(),saveat=0.1)
```

Side note, using `@derivatives`

as in the example gives a deprecation warning. With this setup, the error I get is

```
julia> prob = discretize(pdesys,discretization)
ERROR: LoadError: type Int64 has no field val
```

This is with Julia v1.6.1 and

```
(diffeq) pkg> st
Status `~/dev/learning/diffeq/Project.toml`
[9fdde737] DiffEqOperators v4.24.0
[961ee093] ModelingToolkit v5.16.0
[1dea7af3] OrdinaryDiffEq v5.52.4
[91a5bcdd] Plots v1.12.0
```

Thank you in advance for any help.

Edit: This error seems similar to this one, however, that issue was traced to using complex numbers, which does not seem to be the case here.