[Python] Pandas .to_csv() gives "No engine for filetype: 'csv'"

Basically, the title. The pd.Dataframe.to_csv() method works fine on my machine outside of the app code, but if I try to run it in a pyqt5 gui, I get the following error: No engine for filetype: 'csv'. Below is the function that causes the error. Note: if the user gives an .xlsx file, the .to_excel() function works fine. The file name the function uses is passed through a QFileDialog object. Here is the function causing the problem.

def writeToFile(self, file, rows):

try:

new_df = pd.DataFrame(rows.items(), columns = ['Company Name', 'File Number'] )

if self.stack.currentIndex() == 0:

if file[-4:] == '.csv':

df = pd.read_csv(file)
new_df = pd.concat([df, new_df])

else:

df = pd.read_excel(file)
new_df = pd.concat([df, new_df])

if file[-4] == '.csv':

new_df.to_csv(file, index = False, compression = None)

else:

new_df.to_excel(file)

if self.flag_uids == []:

return "All emails were written to a file"

else:
s = "All emails possible were written to a file. There were " + str(len(self.flag_uids)) + " messages unable to be interpreted"
return s

except Exception as e:

return e

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