"tahoe get" attribute error

[meejah]

I get an exception ending with:

    return self.currentContext().callWithContext(ctx, func, *args, **kw)                                 
  File "/home/meejah/work/leastauthority/src/tahoe-lafs/venv/local/lib/python2.7/site-packages/twisted/python/context.py", line 85, in callWithContext
    return func(*args,**kw)
  File "/home/meejah/work/leastauthority/src/tahoe-lafs/src/allmydata/scripts/cli.py", line 511, in get  
    rc = tahoe_get.get(options)
  File "/home/meejah/work/leastauthority/src/tahoe-lafs/src/allmydata/scripts/tahoe_get.py", line 36, in get
    outf = outf.buffer
exceptions.AttributeError: 'file' object has no attribute 'buffer'         

When running tahoe -d testgrid/bob get URI:CHK:<the cap> in a local test-grid.