[Kwant] Conductance plot for continuum.discretize hamiltonian

[declanburke95]

I defined a QAHE hamiltonian:

ham = ("alpha * (k_x * sigma_x - k_y * sigma_y)"
            "+ (m + beta * kk) * sigma_z"
            "+ (gamma * kk + U) * sigma_0")
subs = {"kk": "k_x**2 + k_y**2"}

Upon using the continuum.discretize  I was able to plot bands etc but am 
struggling to plot the conductance. Using the method from the kwant 
documentation I am getting the following error:

UserCodeError: Error occurred in user-supplied value function "onsite".
See the upper part of the above backtrace for more information.

I am new to kwant so any help would be appreciated!