[Kwant] Zero velocity mode problem when calculating transmission

[于国栋]

Dear Kwant users:
I have meet a problem when calculating the transport properties of zigzag-edged graphene. At energy 0eV, kwant shows me the error: numpy.linalg.linalg.LinAlgError: singular matrix.
After applying the staggered potential, I got this error: RuntimeError: Found a mode with zero or close to zero velocity, because there are two almost flat subbands.
I want to know whether kwant can not handle the case when lead exists the flat subbands.

Thanks a lot
Guodong Yu