Shapely-1.5.12 issue?

[Revo86]

I'm struggling with imposm, after I've entered the following command:

$ ~/venv/bin/imposm --concurrency 2 --read --overwrite-cache --write -d osm -U openstreetmap -m /home/openstreetmap/OSM/imposm-mapping-bus-2.py --deploy-production-tables ~/nordrhein-westfalen-latest.osm.bz2

Enabling Shapely speedups.
Traceback (most recent call last):
  File "/home/openstreetmap/venv/bin/imposm", line 9, in <module>
    load_entry_point('imposm==2.6.0', 'console_scripts', 'imposm')()
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/pkg_resources.py", line 353, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2321, in load_entry_point
    return ep.load()
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2048, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/imposm/app.py", line 39, in <module>
    import imposm.mapping
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/imposm/mapping.py", line 18, in <module>
    import imposm.geom
  File "/home/openstreetmap/venv/local/lib/python2.7/site-packages/imposm/geom.py", line 51, in <module>
    SHAPELY_SUPPORTS_BUFFER = shapely.geos.geos_capi_version >= (1, 6, 0)
AttributeError: 'module' object has no attribute 'geos_capi_version'

What am I missing? Do I need another GEOS version?