locale で 'ValueError: unknown locale: UTF-8' というエラーがでた

locale.py

def getdefaultlocale(envvars=('LC_ALL', 'LC_CTYPE', 'LANG', 'LANGUAGE')):

    # 略

    import os
    lookup = os.environ.get
    for variable in envvars:
        localename = lookup(variable,None)
        if localename:
            if variable == 'LANGUAGE':
                localename = localename.split(':')[0]
            break
    else:
        localename = 'C'
    return _parse_localename(localename)