I agree that test further above is right (that the real part is I*i*r*im(x)) and since you are differentiating wrt to `i` and not `im(i)` then your derivative looks right. Perhaps the person that had the previous test was thinking that `diff(i)` meant `diff(im(i))` in which case the result that is there would be correct...but SymPy doesn't allow for that differentiation:

```
>>> var('i',imaginary=True)
i>>> i.diff(im(i))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sympy\core\expr.py", line 2700, in diff
    return Derivative(self, *new_symbols, **assumptions)
  File "sympy\core\function.py", line 925, in __new__
    Can\'t differentiate wrt the variable: %s, %s''' % (v, count)))
ValueError:
Can't differentiate wrt the variable: -I*i, 1
```