why lemmatizer fails?

def wrangling_doc(doc): tokens=doc.split() re_punc = re.compile('[%s]' % re.escape(string.punctuation)) # remove punctuation from each word tokens = [re_punc.sub('', w) for w in tokens] # remove remaining tokens that are not alphabetic tokens = [word for word in tokens if word.isalpha()] # filter out short tokens tokens = [word for word in tokens if len(word) > 4] #lowercase all words tokens = [word.lower() for word in tokens] # Lemmatize the tokens tokens = [word.lemmatizer() for word in tokens]

return tokens