无法将提取的POS标记名词短语附加到pandas数据框

import pandas as pd
import nltk

# 提取POS标记名词短语
def extract_noun_phrases(text):
    # 使用NLTK库进行POS标记和名词短语提取
    tokens = nltk.word_tokenize(text)
    tagged = nltk.pos_tag(tokens)
    grammar = "NP: {<DT>?<JJ>*<NN>}"
    cp = nltk.RegexpParser(grammar)
    result = cp.parse(tagged)
    noun_phrases = [subtree.leaves() for subtree in result.subtrees() if subtree.label() == 'NP']
    return [' '.join([word for word, tag in phrase]) for phrase in noun_phrases]

# 创建一个示例数据框
data = {'text': ['This is a sample sentence.', 'Another sentence with a noun phrase.']}
df = pd.DataFrame(data)

# 创建新列
df['noun_phrases'] = ''

# 遍历数据框的每一行
for index, row in df.iterrows():
    # 提取名词短语
    phrases = extract_noun_phrases(row['text'])
    # 将提取的名词短语附加到新列
    df.at[index, 'noun_phrases'] = phrases

# 打印结果
print(df)