What does the error message "state/source dependence" mean?

Problem description: 

My simulation will not even start and this message appears, why does this happen?


PLECS models physical systems using state-space equations of the form

dx/dt = A·x + B·u

where x is a vector of state variables (i.e. inductor currents and capacitor voltages for electrical systems) and u is a vector of input variables (i.e. voltage and current sources for electrical systems). In every simulation step, the solver takes the present values of x and u as input variables, calculates the state derivatives dx/dt and uses them to determine the state values for the following simulation step.

A capacitor that is connected directly in parallel with a voltage source is described by the equation

0 = xcap + usrc

and so creates a direct dependency between a state variable and a source variable. This is not possible using the approach described above and so PLECS reports an error due to a state/source dependency. To resolve this problem, you could insert a small resistance in series with either the capacitor or the voltage source. However, the resistance must be chosen carefully so that it does not influence the system dynamics too much (if it is too large) or create numerical problems (if it is too small).

For the same reason it is not permitted to connect an inductor in series with a current source as this would create a direct dependency between the inductor current (the state variable) and the source current (the input variable).