'Test that expressions that evaluate to NaN are not marked as correct.'

def test_grade_nan(self):

    sample_dict = {'x': (1, 2)}
    problem = self.build_problem(sample_dict=sample_dict, num_samples=10, tolerance='1%', answer='x')
    input_formula = '10*x    +    0*1e999'
    self.assert_grade(problem, input_formula, 'incorrect')
    input_formula = 'x    +    0*1e999'
    self.assert_grade(problem, input_formula, 'incorrect')
