Example error regarding piqs

[Htxxxg]

Hey:
I am trying to learn PIQS, but I encountered some unpleasant errors while running the examples.
The example path is: https://nbviewer.org/github/qutip/qutip-notebooks/blob/master/examples/piqs-overview.ipynb .
The location is 1. N Qubits Dynamics.
When running to the In [4] code block, an error occurred: RuntimeError: Factor is exactly singular.

[Htxxxg]

I was able to successfully run on my personal laptop when setting the system dimension N=5, but when the dimension increases, I will report an error: singular matrix. Can a system with a larger N be calculated using a more powerful computer?
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