diff --git a/pep-3333.txt b/pep-3333.txt
--- a/pep-3333.txt
+++ b/pep-3333.txt
@@ -201,7 +201,8 @@ In short: where you see the word "string
 to a "native" string, i.e., an object of type ``str``, whether it is 
 internally implemented as bytes or unicode.  Where you see references 
 to "bytestring", this should be read as "an object of type ``bytes`` 
-under Python 3, or type ``str`` under Python 2".
+under Python 3, or type ``str`` under Python 2".  Instances of ``str``
+subclasses are *not* valid; only the built-in ``str`` type is.
 
 And so, even though HTTP is in some sense "really just bytes", there
 are  many API conveniences to be had by using whatever Python's