Attempted write to NoneType

[pnorman]

against 3531bd4

./addressmerge.py -d xapi -w berkeley.wkt berkeley_address.osm berkeley_out.osm

...

Traceback (most recent call last):
  File "./addressmerge.py", line 538, in <module>
    source.output_osc(existing, args.osc)
  File "./addressmerge.py", line 470, in output_osc
    f.write('<?xml version="1.0"?>\n<osmChange version="0.6" upload="false" generator="addressmerge">\n')
AttributeError: 'NoneType' object has no attribute 'write'