Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 57068)
+++ Lib/io.py	(working copy)
@@ -588,7 +588,9 @@
 
     def __init__(self, initial_bytes=None):
         buffer = b""
-        if initial_bytes is not None:
+        if isinstance(initial_bytes, str):
+            buffer += bytes(initial_bytes)
+        elif initial_bytes is not None:
             buffer += initial_bytes
         self._buffer = buffer
         self._pos = 0