I had the same problem using version 10.5. I am able to export the CSS' but cannot re-import them using the same file because of this error.
I was able to find a work around by removing the trailing commas in the CSV file for the entries that don't have as many partitions. When I re-imported the same .tar file, it didn't read the empty spaces as NULL.
Just remove the commas for the empty fields and try it again. Let me know if it works for you as well.
Thanks,
Giorgio