Unable to build documentation (ValueError: unknown locale: UTF-8)

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When I try to build the sage documentation I get an error. What am I doing wrong? The same error appears when I try

make doc-clean && make

Here is a transcript:

amri@as154:~/sage$ sage -docbuild
Traceback (most recent call last):
  File "/home/amri/sage/src/doc/common/builder.py", line 16, in <module>
    import sphinx.cmdline
  File "/home/amri/sage/local/lib/python2.7/site-packages/Sphinx-1.2.2-py2.7.egg/sphinx/cmdline.py", line 18, in <module>
    from docutils.utils import SystemMessage
  File "/home/amri/sage/local/lib/python2.7/site-packages/docutils/utils/__init__.py", line 20, in <module>
    import docutils.io
  File "/home/amri/sage/local/lib/python2.7/site-packages/docutils/io.py", line 18, in <module>
    from docutils.utils.error_reporting import locale_encoding, ErrorString, ErrorOutput
  File "/home/amri/sage/local/lib/python2.7/site-packages/docutils/utils/error_reporting.py", line 47, in <module>
    locale_encoding = locale.getlocale()[1] or locale.getdefaultlocale()[1]
  File "/home/amri/sage/local/lib/python/locale.py", line 543, in getdefaultlocale
    return _parse_localename(localename)
  File "/home/amri/sage/local/lib/python/locale.py", line 475, in _parse_localename
    raise ValueError, 'unknown locale: %s' % localename
ValueError: unknown locale: UTF-8