diff --git a/Lib/base64.py b/Lib/base64.py
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -28,6 +28,7 @@
 
 
 bytes_types = (bytes, bytearray)  # Types acceptable as binary data
+ascii_types = bytes_types + (str,)
 
 
 def _translate(s, altchars):
@@ -79,10 +80,10 @@
     discarded prior to the padding check.  If validate is True,
     non-base64-alphabet characters in the input result in a binascii.Error.
     """
-    if not isinstance(s, bytes_types):
+    if not isinstance(s, ascii_types):
         raise TypeError("expected bytes, not %s" % s.__class__.__name__)
     if altchars is not None:
-        if not isinstance(altchars, bytes_types):
+        if not isinstance(altchars, ascii_types):
             raise TypeError("expected bytes, not %s"
                             % altchars.__class__.__name__)
         assert len(altchars) == 2, repr(altchars)
@@ -211,7 +212,7 @@
     the input is incorrectly padded or if there are non-alphabet
     characters present in the input.
     """
-    if not isinstance(s, bytes_types):
+    if not isinstance(s, ascii_types):
         raise TypeError("expected bytes, not %s" % s.__class__.__name__)
     quanta, leftover = divmod(len(s), 8)
     if leftover:
@@ -220,7 +221,7 @@
     # False, or the character to map the digit 1 (one) to.  It should be
     # either L (el) or I (eye).
     if map01 is not None:
-        if not isinstance(map01, bytes_types):
+        if not isinstance(map01, ascii_types):
             raise TypeError("expected bytes, not %s" % map01.__class__.__name__)
         assert len(map01) == 1, repr(map01)
         s = _translate(s, {b'0': b'O', b'1': map01})
@@ -292,7 +293,7 @@
     s were incorrectly padded or if there are non-alphabet characters
     present in the string.
     """
-    if not isinstance(s, bytes_types):
+    if not isinstance(s, ascii_types):
         raise TypeError("expected bytes, not %s" % s.__class__.__name__)
     if casefold:
         s = s.upper()