diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index 4c36e4b..f4b4264 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -56,6 +56,11 @@
 {
 	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++;