Be careful using `1/18` here. Python will evaluate that to 0 in Python 2 and 0.05555 in Python 3. See http://docs.sympy.org/0.7.3/tutorial/gotchas.html#two-final-notes-and and http://docs.sympy.org/0.7.3/gotchas.html#python-numbers-vs-sympy-numbers.

Basically, whenever you have `number/number`, you need to have `S(number)/number` instead. Maybe that is why this test fails?