calibredb exception (another!)

[ZioNemo]

I'm creating an empty db and adding a few (3, for the time being) custom columns.

I get the following errors from my program (java, using ProcessBuilder to spawn calibredb):

Command '[/usr/bin/calibredb, add_custom_column, --with-library=/mnt/fileserver/Store/Store/eBooks/Calibre, ddlib, DD_Library, series]' failed.
[[err]] Traceback (most recent call last):
[[err]]   File "site.py", line 58, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 1475, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 771, in command_add_custom_column
[[err]]   File "site-packages/calibre/library/cli.py", line 722, in do_add_custom_column
[[err]] TypeError: %d format: a number is required, not NoneType
Command '[/usr/bin/calibredb, add_custom_column, --with-library=/mnt/fileserver/Store/Store/eBooks/Calibre, ddlink, DD_Post, text]' failed.
[[err]] Traceback (most recent call last):
[[err]]   File "site.py", line 58, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 1475, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 771, in command_add_custom_column
[[err]]   File "site-packages/calibre/library/cli.py", line 722, in do_add_custom_column
[[err]] TypeError: %d format: a number is required, not NoneType
Command '[/usr/bin/calibredb, add_custom_column, --with-library=/mnt/fileserver/Store/Store/eBooks/Calibre, ed2k, ed2k, text]' failed.
[[err]] Traceback (most recent call last):
[[err]]   File "site.py", line 58, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 1475, in main
[[err]]   File "site-packages/calibre/library/cli.py", line 771, in command_add_custom_column
[[err]]   File "site-packages/calibre/library/cli.py", line 722, in do_add_custom_column
[[err]] TypeError: %d format: a number is required, not NoneType

Anyways it seems everythinh went ok :

mcon@vmrunner:/mnt/fileserver/Store/Store/eBooks/Calibre$ /usr/bin/calibredb custom_columns "--with-library=/mnt/fileserver/Store/Store/eBooks/Calibre" -d
ddlib

{u'datatype': u'series',
 u'display': {},
 u'editable': True,
 u'is_multiple': False,
 u'label': u'ddlib',
 u'multiple_seps': {},
 u'name': u'DD_Library',
 u'normalized': True,
 u'num': 1}


ed2k

{u'datatype': u'text',
 u'display': {},
 u'editable': True,
 u'is_multiple': False,
 u'label': u'ed2k',
 u'multiple_seps': {},
 u'name': u'ed2k',
 u'normalized': True,
 u'num': 3}


ddlink

{u'datatype': u'text',
 u'display': {},
 u'editable': True,
 u'is_multiple': False,
 u'label': u'ddlink',
 u'multiple_seps': {},
 u'name': u'DD_Post',
 u'normalized': True,
 u'num': 2}


mcon@vmrunner:/mnt/fileserver/Store/Store/eBooks/Calibre$

What am I doing wrong?
I will ignore the error for now, but I'm a bit uneasy...

Note: The first line contains the arguments passed to the subprocess, so my program is equivalent to:

/usr/bin/calibredb "add_custom_column" "--with-library=/mnt/fileserver/Store/Store/eBooks/Calibre" "ddlib" "DD_Library" "series"
/usr/bin/calibredb "add_custom_column" "--with-library=/mnt/fileserver/Store/Store/eBooks/Calibre" "ddlink" "DD_Post" "text"
/usr/bin/calibredb "add_custom_column" "--with-library=/mnt/fileserver/Store/Store/eBooks/Calibre" "ed2k" "ed2k" "text"