applying function to all columns in an numpy array

I am extremely new to numpy.

Just wondering why does this not work.

print items['description'] 

Yields

0                            Продам Камаз 6520 20 тонн
1                                      Весь в тюнинге.
2    Телефон в хорошем состоянии, трещин и сколов н...
3    Отличный подарок на новый год от "китайской ap...
4        Лыжные ботинки в хорошем состоянии, 34 размер
Name: description, dtype: object

Trying to apply this method to all the rows in this col.

items['description'] = vectorize_sentence(items['description'].astype(str))

This is the function definition for vectorize sentence.

def vectorize_sentence(self, sentence):
    # Tokenize 
    print 'sentence', sentence

    tkns = self._tokenize(sentence)
    vec = None
    for tkn in tkns: 
        print 'tkn', tkn.decode('utf-8')
        print type(tkn)
        if self.model[tkn.decode('utf-8')]:
            vec = sum(vec, self.model[tkn.decode('utf-8')])
    #vec = sum([self.model[x] for x in tkns if x in self.model])
    #print vec
def _tokenize(self, sentence):
    return sentence.split(' ')

Error Message:

AttributeError: 'Series' object has no attribute 'split'