diff --git a/Lib/base64.py b/Lib/base64.py
index 1b9f560cc66..e100e0fab02 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -50,7 +50,7 @@ def b64encode(s, altchars=None):
     The encoded byte string is returned.
     """
     if not isinstance(s, bytes):
-        s = bytes(s)
+        s = bytes(s, "ascii")
     # Strip off the trailing newline
     encoded = binascii.b2a_base64(s)[:-1]
     if altchars is not None: