Oops, try again. grades_variance([9, 9, 5, 9, 0, 5, 4]) returned 0.61197657 instead of the expected: 9.8367346939

I followed all the instructions! What's wrong?

```
def grades_variance(scores):
average = grades_average(scores)
variance = 0
for score in scores:
variance += (average - score) ** 2
variance /= len(scores)
return variance
print grades_variance(grades)
```