Example error regarding piqs

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.
Thanks