unable to build the doc reference html

edit

./sage --docbuild reference html or make doc-clean && make I work on the ticket t/20154/train_tracks

give error:

[polynomia] WARNING: intersphinx inventory '/home/dominique/projets/sage/local/share/doc/sage/inventory/en/reference/objects.inv' not fetchable due to <type 'exceptions.IOError'>: [Errno 2] No such file or directory: '/home/dominique/projets/sage/local/share/doc/sage/inventory/en/reference/objects.inv'
[polynomia] None:3: WARNING: citation not found: CMR05
Error building the documentation.
Traceback (most recent call last):
  File "/home/dominique/projets/sage/local/lib/python/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/home/dominique/projets/sage/local/lib/python/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/dominique/projets/sage/local/lib/python2.7/site-packages/sage_setup/docbuild/__main__.py", line 2, in <module>
    main()
  File "/home/dominique/projets/sage/local/lib/python2.7/site-packages/sage_setup/docbuild/__init__.py", line 1629, in main
    builder()
  File "/home/dominique/projets/sage/local/lib/python2.7/site-packages/sage_setup/docbuild/__init__.py", line 495, in _wrapper
    x.get(99999)
  File "/home/dominique/projets/sage/local/lib/python/multiprocessing/pool.py", line 567, in get
    raise self._value
OSError: [polynomia] WARNING: intersphinx inventory '/home/dominique/projets/sage/local/share/doc/sage/inventory/en/reference/objects.inv' not fetchable due to <type 'exceptions.IOError'>: [Errno 2] No such file or directory: '/home/dominique/projets/sage/local/share/doc/sage/inventory/en/reference/objects.inv'

Note: incremental documentation builds sometimes cause spurious error messages. To be certain that these are real errors, run "make doc-clean" first and try again.