Only semi-explicit constant mass matrices are currently supported

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Following is a sample code that I got from examples folder in the ModelingToolkit.jl. I added the last two lines to formulate the Problem. What are the possible reasons for the error in code " Only semi-explicit constant mass matrices are currently supported. Faulty equation: source₊p₊v(t) ~ resistor₊p₊v(t).". There are 26 states and 4 parameters whose default values were taken for the simulation.

include("electrical_components.jl")

@named source = ConstantVoltage(V=10.0)
@named resistor = Resistor(R=1.0)
@named inductor1 = Inductor(L=1.0e-2)
@named inductor2 = Inductor(L=2.0e-2)
@named ground = Ground()

eqs = [
       connect(source.p, resistor.p)
       connect(resistor.n, inductor1.p)
       connect(inductor1.n, inductor2.p)
       connect(source.n, inductor2.n, ground.g)
      ]

@named ll_model = ODESystem(eqs, t)
ll_model = compose(ll_model, [source, resistor, inductor1, inductor2, ground])
sys =ll_model

tspan = (0.0,100.0)
prob = ODEProblem(sys,[],tspan,[])