GDAL raster shifting with python error: 'NoneType' object has no attribute 'GetGeoTransform'

I’m trying to shift a georeferenced raster by 0.5m N and 0.5m W in QGIS using the python console:

from osgeo import gdal

# Open in read/write mode
rast_src = gdal.Open('test_raster.tif', 1)

# Get affine transform coefficients
gt = rast_src.GetGeoTransform()

# Convert tuple to list, so we can modify it
gtl = list(gt)
gtl[0] -= 0.5  # Move west 0.5 m
gtl[3] += 0.5 # Move north 0.5 m

# Save the geotransform to the raster
rast_src.SetGeoTransform(tuple(gtl))
rast_src = None # equivalent to save/close

However line #7 (gt = rast_src.GetGeoTransform()) throws an AttributeError: ‘NoneType’ object has no attribute ‘GetGeoTransform’. The installed GDAL version is 1.11.0. The raster seems fine. So what’s wrong?