Is there some requirements for the .xlsx file?

[Kidsunbo]

In the document, it's said that this library can only work with .xlsx but not .xls. I have a .xls file opened and save as .xlsx, then I read the data but the return value is always empty.But for some other .xlsx files, the program works. I've no idea how to solve this. The code I wrote was as easy as the samples, but it didn't work with some .xlsx files ... Have you encountered sucn a problem? Help~~~

[VSRonin]

How do you open and save the .xls file?