The return type of email.header.decode_header is not consistent. When there are encoded parts the return type is a list of (bytes, charset or None) (Note that the documentation says it is a list of (str, charset)). However, when there are no encoded parts the return type is [(str, None)]. Note that, at the end of the function, there is a routine that converts everything to bytes.

Compare:
In [01]: email.header.decode_header('=?UTF-8?Q?foo?=bar')
Out[01]: [(b'foo', 'utf-8'), (b'bar', None)]

In [02]: email.header.decode_header('foobar')
Out[02]: [('foobar', None)]