diff -u -r old/Include/stringobject.h new/Include/stringobject.h
--- old/Include/stringobject.h	2003-07-01 13:15:21.000000000 -0700
+++ new/Include/stringobject.h	2004-10-25 16:01:32.381195821 -0700
@@ -80,6 +80,11 @@
 /* Macro, trading safety for speed */
 #define PyString_AS_STRING(op) (((PyStringObject *)(op))->ob_sval)
 #define PyString_GET_SIZE(op)  (((PyStringObject *)(op))->ob_size)
+#define PyString_INVALIDATE(op) \
+    do { \
+        ((PyStringObject *)(op))->ob_shash = -1; \
+        ((PyStringObject *)(op))->ob_sstate = SSTATE_NOT_INTERNED; \
+    } while (0)
 
 /* _PyString_Join(sep, x) is like sep.join(x).  sep must be PyStringObject*,
    x must be an iterable object. */
diff -u -r old/Python/ceval.c new/Python/ceval.c
--- old/Python/ceval.c	2004-10-10 19:40:50.000000000 -0700
+++ new/Python/ceval.c	2004-10-25 16:13:59.172247438 -0700
@@ -4284,6 +4284,8 @@
 		/* copy 'w' into the newly allocated area of 'v' */
 		memcpy(PyString_AS_STRING(v) + v_len,
 		       PyString_AS_STRING(w), w_len);
+		/* we must also invalidate the cached hash value! */
+		PyString_INVALIDATE(v);
 		return v;
 	}
 	else {