When trying to sumerize an article from the web using nltk, the error: argument of type Nontype is not iterable has been thrown

I am following an online python course and the exercise is to summarize an article from a website using nltk. In the following code i am setting up a class with an inbuilt function to summarize the text i have extracted from the url.

class FrequencySummarizer:
    def __init__(self,min_cut=0.1,max_cut=0.9):
        self._min_cut = min_cut
        self._max_cut = max_cut
        self._stopwords = set(stopwords.words('english')+
                             list(punctuation))
    def __compute__frequencies(self,word_sent,customStopWords=None):
        freq = defaultdict(int)
        if customStopWords is None:
            stopwords = set(self._stopwords)
        else:
            stopwords = set(customStopWords).union(self._stopwords)
            for sentence in word_sent:
                for word in sentence:
                    if word not in stopwords:
                        freq[word] += 1
            m = float(max(freq.values()))
            for word in freq.keys():
                freq[word]=freq[word]/m
                if freq[word] >= self._max_cut or freq[word] <= self._min_cut:
                    del freq[word]
            return freq
    def summarize(self, text ,n):
        
        sentences= sent_tokenize(text)
        
        assert n<=len(sentences)
    
        word_sent= [word_tokenize(s.lower()) for s in sentences]
    
        self._freq = self.__compute__frequencies(word_sent)
        ranking = defaultdict(int)
        for i,sentences in enumerate(word_sent): 
            for word in sentences:
                if word in self._freq:
                    ranking[i] += self._freq[word]
        sentences_index = nlargest(n,ranking, key=rankings.get)
        return [sents[j] for j in sentences_index]   

The error thrown looks like this:

TypeError                                 Traceback (most recent call last)
<ipython-input-44-a60e7a2fe76f> in <module>
      1 fs = FrequencySummarizer()
----> 2 summary = fs.summarize(textOfUrl[0],3)

<ipython-input-43-9a4a271838d4> in summarize(self, text, n)

     33         for i,sentences in enumerate(word_sent):
     34             for word in sentences:
---> 35                 if word in self._freq:
     36                     ranking[i] += self._freq[word]
     37         sentences_index = nlargest(n,ranking, key=rankings.get)

TypeError: argument of type 'NoneType' is not iterable

I believe the error occurs because I have incorrectly set up the self._freq variable, this could be in the line where i define the word_sent variable or in the __compute__frequencies function, I can't tell.