Traceback (most recent call last):

File "python", line 30, in

File "python", line 26, in grades_std_deviation

TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'float'

I can't figure out what is wrong here. I have it exactly as the instructions say, and I've looked through the relevant forum posts to confirm, but I still get an error.

```
grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]
def print_grades(grades):
for grade in grades:
print grade
def grades_sum(grades):
total = 0
for grade in grades:
total += grade
return total
def grades_average(grades):
sum_of_grades = grades_sum(grades)
average = sum_of_grades / float(len(grades))
return average
def grades_variance(scores):
average = grades_average(scores)
variance = 0
for score in scores:
variance += (average - score) ** 2
return variance / (len(scores))
def grades_std_deviation(variance):
return variance ** 0.5
variance = grades_variance(grades)
print grades_std_deviation(grades)
```