Stuck in the third project

My Code:

common_brands_mean_price = {}

for brand in common_brands:
brand = autos[autos[‘brand’] == brand]
mean_price = brand[‘price_dollar’].mean()
common_brands_mean_price[brand] = int(mean_price)

Replace this line with your code

What I expected to happen:

What actually happened: 

Replace this line with the output/error

<ipython-input-98-44b68952cdde> in <module>()
common_brands = autos['brand'].value_counts(normalize=True)[:4].index
      4     brand = autos[autos['brand'] == brand]
      5     mean_price = brand['price_dollar'].mean()
----> 6     common_brands_mean_price[brand] = int(mean_price)
      8 common_brands_mean_price

/dataquest/system/env/python3/lib/python3.4/site-packages/pandas/core/ in __hash__(self)
   1043     def __hash__(self):
   1044         raise TypeError('{0!r} objects are mutable, thus they cannot be'
-> 1045                         ' hashed'.format(self.__class__.__name__))
   1047     def __iter__(self):

TypeError: 'DataFrame' objects are mutable, thus they cannot be hashe

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I'm stuck in this part of the third guided project.

Could someone explain to me what am I missing?

Thanks for your help