diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 668d239..f78788e 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -4000,6 +4000,13 @@
     	return NULL;
     }
 
+    /* Grrrr.  A codec may be invoked to convert str objects to
+     * Unicode, and so it's possible to call back into Python code
+     * during PyUnicode_FromObject(), and so it's possible for a sick
+     * codec to change the size of fseq (if seq is a list).  Therefore
+     * we have to keep refetching the size -- can't assume seqlen
+     * is invariant.
+     */
     seqlen = PySequence_Fast_GET_SIZE(fseq);
     /* If empty sequence, return u"". */
     if (seqlen == 0) {
@@ -4029,6 +4036,8 @@
 	        goto onError;
 	    sep = PyUnicode_AS_UNICODE(internal_separator);
 	    seplen = PyUnicode_GET_SIZE(internal_separator);
+	    /* In case PyUnicode_FromObject() mutated seq. */
+	    seqlen = PySequence_Fast_GET_SIZE(fseq);
         }
     }
 
@@ -4057,6 +4066,9 @@
 	    goto onError;
 	/* We own a reference to item from here on. */
 
+	/* In case PyUnicode_FromObject() mutated seq. */
+	seqlen = PySequence_Fast_GET_SIZE(fseq);
+
         /* Make sure we have enough space for the separator and the item. */
 	itemlen = PyUnicode_GET_SIZE(item);
 	new_res_used = res_used + itemlen;