Python df.apply（lambda:x.lower（））：'；功能'；对象没有属性'；下'；

我已经用Jupyter编写了这段代码，但收到一条错误消息：

tokenizer = RegexpTokenizer (r'\w+')

career_df['How could the conversation have been more useful?']= career_df['How could the conversation have been more useful?'].apply(lambda x:tokenizer.tokenize(x.lower()))

错误是

AttributeError:“function”对象没有属性“lower”

lambda表达式中的“x”在应该是字符串时充当函数。 尝试：

你应该回答你的问题，指出你导入了哪些库，这样我们就不用猜了。理想情况下，代码应该构成一个完整的代码。

  career_df['How could the conversation have been more useful?']= career_df.apply(lambda x:tokenizer.tokenize(x['How could the conversation have been more useful?'].lower()))