Variable is not a function of independent variable error for trivial example

Specific Domains Modelling & Simulations
1

I am trying out the ModelingToolkit.jl package and I get this weird error when defing the classical harmonic oscillator with the following code

using ModelingToolkit,
@variables t x(t)
@parameters ω₀
Dₜ = Differential(t)
@named sys = ODESystem([Dₜ(Dₜ(x)) + ω₀^2*x ~ 0])

It produces the error

ArgumentError: Variable x(t) is not a function of independent variable (ω₀^2)*x(t).

I do not understand the where the error comes from. Just putting the second term on the other side gets rid of the error, i.e.

using ModelingToolkit,
@variables t x(t)
@parameters ω₀
Dₜ = Differential(t)
@named sys = ODESystem([Dₜ(Dₜ(x)) ~ - ω₀^2*x])

runs just fine. My apologies if this error comes from my lack of understanding how the package must be used.

2

If you explicitly provide the independent variable your first formulation works.

@named sys = ODESystem([Dₜ(Dₜ(x)) + ω₀^2*x ~ 0], t)
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