Example code producing the bug:
`from quantities import C, N, m, A, mm, V, F,
pF, H, ohm, Hz, MHz, S, s, kg, mS, rad
import numpy as np
from numpy import pi, exp

epsilon0 = 8.854187817 * 10**(-12) * C2/(N*m2)
mu0 = 4pi10**(-7) * N/(A**2)

def R_outer(r_center, Z_0, epsilonr):
epsilon = epsilon0epsilonr
return r_center(exp(2piZ_0/np.sqrt(mu0/epsilon)) - 1)

Z_0 = 75ohm
r_center = 0.6mm
R_outer(r_center, Z_0, 2.25)`

Running that code gives this error:

If I print out the expression inside the exp function, I get this:

The issue seems to be that it's treatingohm as if it were a base SI unit, rather than splitting it up into the base units and cancelling out common units, as it did for Coulombs and Newtons, and as the documentation for the simplify method says it should. It's easy to check that all those units do in fact cancel out by rewriting ohms in base units.