关于python：re.sub错误与“预期的字符串或字节类对象”

re.sub erroring with “Expected string or bytes-like object”

我已经阅读了多篇关于这个错误的文章，但我仍然无法理解。当我试图循环我的函数时：

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def fix_Plan(location):
letters_only = re.sub("[^a-zA-Z]", # Search for all non-letters
"", # Replace all non-letters with spaces
location) # Column and row to search

words = letters_only.lower().split()
stops = set(stopwords.words("english"))
meaningful_words = [w for w in words if not w in stops]
return ("".join(meaningful_words))

col_Plan = fix_Plan(train["Plan"][0])
num_responses = train["Plan"].size
clean_Plan_responses = []

for i in range(0,num_responses):
clean_Plan_responses.append(fix_Plan(train["Plan"][i]))

错误如下：

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Traceback (most recent call last):
File"C:/Users/xxxxx/PycharmProjects/tronc/tronc2.py", line 48, in <module>
clean_Plan_responses.append(fix_Plan(train["Plan"][i]))
File"C:/Users/xxxxx/PycharmProjects/tronc/tronc2.py", line 22, in fix_Plan
location) # Column and row to search
File"C:\Users\xxxxx\AppData\Local\Programs\Python\Python36\lib
e.py", line 191, in sub
return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or bytes-like object