Python Band.SetNoDataValue(), sets all bands at once

Can someone tell me what I am doing wrong? I want to set bands 1 and three to 255, and band 2 to 0. The code below only should only set band 2, but gdalinfo shows that all bands get set to 0. Ubuntu 14.04, gdal 1.11.0. Thanks!

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import sys
import os.path
import gdal

input = sys.argv[1]

dataset = gdal.Open( input, gdal.GA_Update )
if dataset is None:
    print 'Unable to open', input, 'for writing'
    sys.exit(1)    
band = dataset.GetRasterBand(2)
band.SetNoDataValue(0.0)
band.FlushCache()
band = None
dataset = None

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$ gdalinfo test.tif
...
Upper Right (-11546254.150, 4881966.253) 
Lower Right (-11546254.150, 4244288.152) 
Center      (-11950987.074, 4563127.202) 
Band 1 Block=15107x1 Type=Byte, ColorInterp=Red
  NoData Value=0
Band 2 Block=15107x1 Type=Byte, ColorInterp=Green
  NoData Value=0
Band 3 Block=15107x1 Type=Byte, ColorInterp=Blue
  NoData Value=0