execute str.upper() on multi-type data type column will change number data to NA
Version : 0.16.2
OS : Mac

What I think is when use .str need first change compulsory change dtype to str, but in my situation, pandas change my number data to NA.

And when I use .astype('str')  first,  I got the output I need, but I think this may may somebody confused and make some mistake without .astype('str') .
