TypeError in check -t quicks

[jneubert]

Not sure if "quicks" output format is supposed to survive - if not, the following is not relevant.

The following command gives an TypeError:

$ wdmapper check P227 P2428 -i /opt/repec-ras/var/ras/example2/map/gnd_ras_mapping.test.csv -t quicks
# skipping indirect link 124825109 -> Q? -> psn9
Christian von Hirschhausen (GND PRESENT BUT NOT LINKED) P227    "170947386"
Christian von Hirschhausen (GND PRESENT BUT NOT LINKED) P2428   "phi58"
-Q1082503       P227    "170947386"
Traceback (most recent call last):
  File "/usr/bin/wdmapper", line 11, in <module>
    sys.exit(main())
  File "/usr/lib/python2.7/site-packages/wdmapper/cli.py", line 113, in main
    run(*args)
  File "/usr/lib/python2.7/site-packages/wdmapper/cli.py", line 98, in run
    wdmapper.wdmapper(command, **vars(args))
  File "/usr/lib/python2.7/site-packages/wdmapper/wdmapper.py", line 255, in wdmapper
    args.writer.write_delta(delta)
  File "/usr/lib/python2.7/site-packages/wdmapper/format/quicks.py", line 43, in write_delta
    self.edit_link(link,'-')
  File "/usr/lib/python2.7/site-packages/wdmapper/format/quicks.py", line 23, in edit_link
    self.write_command((qid, prop, '"' + link.target + '"'))
TypeError: coercing to Unicode: need string or buffer, NoneType found

LC_ALL and LANG are set according to #22.