[Kwant] issue in class "gauge"

[Sergey Slizovskiy]

Dear Kwant developers,

  I am having a error with "gauge" when I am adding  1D leads (Chains) 
to my 3D system.  (I have both Chain leads, where I do not need gauge, 
of course,

and nirmal leads, where I do need gauge)

  The call:

gauge = kwant.physics.magnetic_gauge(sys)
 
 returns a error:
'not enough values to unpack (expected 2, got 0)',

coming from the

rows, cols = np.array([(i, j)for i, jin syst.graph
                        if i < nand j < n]).transpose()

in
_check_infinite_syst(syst)

in  gauge.py

In my case,  n = 1 for  a simple chain  and so   rows, cols  are empty.

Best wishes,
Sergey