MethodError forgot fixed

[oameye]

When one forget to add the fixed variable one gets a MethodError. However does should be an AssertationError.

@variables ω1, ω2, ωₘ, t, ω, F, γ, λ, x(t), y(t);
eqs = [d(x, t, 2) + (ω1^2 - λ * cos(ωₘ * t)) * x + γ * d(x, t) - λ * cos(ωₘ * t) * y,
    d(y, t, 2) + (ω2^2 - λ * cos(ωₘ * t))y + γ * d(y, t) - λ * cos(ωₘ * t) * x] - [F * cos(ω * t), 0.0]

diff_eq = DifferentialEquation(eqs, [x, y])

add_harmonic!(diff_eq, x, ω) # drive frequency, close to ω1
add_harmonic!(diff_eq, y, ω) # ωₘ will be close ω2 - ω1 to response at the upper sideband
add_harmonic!(diff_eq, x, ω + ωₘ) # ωₘ will be close ω2 - ω1 to response at the upper sideband
add_harmonic!(diff_eq, x, ω - ωₘ)
add_harmonic!(diff_eq, y, ω + ωₘ)
add_harmonic!(diff_eq, y, ω - ωₘ)
add_harmonic!(diff_eq, x, ω + 2 * ωₘ)
add_harmonic!(diff_eq, x, ω - 2 * ωₘ)
add_harmonic!(diff_eq, y, ω + 2 * ωₘ)
add_harmonic!(diff_eq, y, ω - 2 * ωₘ)
add_harmonic!(diff_eq, x, ω + 3 * ωₘ)
add_harmonic!(diff_eq, x, ω - 3 * ωₘ)
add_harmonic!(diff_eq, y, ω + 3 * ωₘ)
add_harmonic!(diff_eq, y, ω - 3 * ωₘ)
add_harmonic!(diff_eq, x, ω + 4 * ωₘ)
add_harmonic!(diff_eq, x, ω - 4 * ωₘ)
add_harmonic!(diff_eq, y, ω + 4 * ωₘ)
add_harmonic!(diff_eq, y, ω - 4 * ωₘ)


fixed = (ω1 => 0.95, ω2 => 1.05, ωₘ => 0.1, γ => 0.005, λ => 0.0, F => 1e-4)   # fixed parameters
varied = ω => range(0.7, 1.3, 100)           # range of parameter values
result = get_steady_states(harmonic_eq, varied, threading=true)

MethodError: no method matching filter(::HarmonicBalance.var"#91#93", ::Pair{Symbolics.Num, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}})
The function `filter` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  filter(::Function, ::DataStructures.LinkedList{T}) where T
   @ DataStructures ~/.julia/packages/DataStructures/95DJa/src/list.jl:81
  filter(::Function, ::DataStructures.MutableLinkedList{T}) where T
   @ DataStructures ~/.julia/packages/DataStructures/95DJa/src/mutable_list.jl:92
  filter(::Any, ::AbstractSet)
   @ Base abstractset.jl:624
  ...


Stacktrace:
 [1] get_steady_states(p::HarmonicEquation, pairs::Pair{Symbolics.Num, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}; kwargs::@Kwargs{threading::Bool})
   @ HarmonicBalance ~/.julia/packages/HarmonicBalance/0J1i1/src/solve_homotopy.jl:180
 [2] top-level scope
   @ In[20]:3