diff --git a/Lib/base64.py b/Lib/base64.py
index 73e9808..e708136 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -58,8 +58,8 @@
     encoded = binascii.b2a_base64(s)[:-1]
     if altchars is not None:
         if not isinstance(altchars, bytes_types):
-            altchars = TypeError("expected bytes, not %s"
-                                 % altchars.__class__.__name__)
+            raise TypeError("expected bytes, not %s"
+                            % altchars.__class__.__name__)
         assert len(altchars) == 2, repr(altchars)
         return _translate(encoded, {'+': altchars[0:1], '/': altchars[1:2]})
     return encoded