diff -r eafff38a56cc Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Fri May 10 05:22:14 2013 +0300
+++ b/Objects/unicodeobject.c	Fri May 10 21:37:20 2013 +0300
@@ -4641,6 +4641,10 @@
 # error C 'long' size should be either 4 or 8!
 #endif
 
+/* Try to decode ASCII-encoded string into PyASCIIObject.
+
+   Returns a number of succesfully decoded characters. */
+
 static Py_ssize_t
 ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
 {
@@ -4648,13 +4652,18 @@
     const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
 
 #if SIZEOF_LONG <= SIZEOF_VOID_P
-    assert(_Py_IS_ALIGNED(dest, SIZEOF_LONG));
-    if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
-        /* Fast path, see in STRINGLIB(utf8_decode) for
-           an explanation. */
+    /* Fast path, see in STRINGLIB(utf8_decode) for
+       an explanation.
+       dest is always aligned on common platforms
+       (if sizeof(PyASCIIObject) is divisible by SIZEOF_LONG).
+       p is aligned on common platforms for bytes and bytearrays
+       (but may not be for a general buffer). */
+    if (sizeof(PyASCIIObject) % SIZEOF_LONG == 0 &&
+        _Py_IS_ALIGNED(p, SIZEOF_LONG)) {
         /* Help register allocation */
         register const char *_p = p;
         register Py_UCS1 * q = dest;
+        assert(_Py_IS_ALIGNED(dest, SIZEOF_LONG));
         while (_p < aligned_end) {
             unsigned long value = *(const unsigned long *) _p;
             if (value & ASCII_CHAR_MASK)