Python 2.7:
>>> from email.header import Header
>>> Header(' ').encode()
''

Python 3.2+ (I didn't check older versions of Python 3):
>>> Header(' ').encode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tim/code/cpython/Lib/email/header.py", line 391, in encode
    value = formatter._str(linesep)
  File "/home/tim/code/cpython/Lib/email/header.py", line 427, in _str
    self.newline()
  File "/home/tim/code/cpython/Lib/email/header.py", line 439, in newline
    self._lines[-1] += str(self._current_line)
IndexError: list index out of range

(Originally reported at https://code.djangoproject.com/ticket/27051)