Hi, Lysianne-
Yes, indeed, it is something that might not be unexpected or a problem. It just that the program wants to make sure that the user is aware and verifies that.
You can use this option to output the mask that the program uses/checks with:
-output_mask_nonnull
:internally, this program checks for where there are
nonnull time series, because we don't like those, in
general. With this flag, the user can output the
determined mask of non-null time series.
... and make sure that these null time series don't appear in bad/unwanted places.
--pt