[BUG] zsv flatten output file not being written

[iamazeem]

$ cat test.csv 
a,b
1,2
3,4
5,6
$ zsv flatten test.csv -o flatten.csv
a,2,4,6
1,,,
3,,,
5,,,
$ ls -l flatten.csv 
-rw-rw-r-- 1 azeem azeem 0 Nov 25 10:44 flatten.csv

With -o, the output is going to stdout, not to file.
The file is being created but the data is not being written to it.