Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(revision 62261)
+++ Objects/stringobject.c	(revision 62262)
@@ -54,6 +54,11 @@
 {
 	register PyStringObject *op;
 	assert(size >= 0);
+	if (size < 0) {
+		PyErr_SetString(PyExc_SystemError,
+		    "Negative size passed to PyString_FromStringAndSize");
+		return NULL;
+	}
 	if (size == 0 && (op = nullstring) != NULL) {
 #ifdef COUNT_ALLOCS
 		null_strings++;