Why do I need ‘b’ to encode a string with Base64?

Following this python example, I encode a string as Base64 with:

>>> import base64
>>> encoded = base64.b64encode(b'data to be encoded')
>>> encoded
b'ZGF0YSB0byBiZSBlbmNvZGVk'

But, if I leave out the leading b:

>>> encoded = base64.b64encode('data to be encoded')

I get the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python32\lib\base64.py", line 56, in b64encode
   raise TypeError("expected bytes, not %s" % s.__class__.__name__)
   TypeError: expected bytes, not str

Why is this?