diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 23de37e..af7a1b1 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -161,6 +161,11 @@
     Py_ssize_t alloc;
 
     assert(size >= 0);
+    if (size < 0) {
+        PyErr_SetString(PyExc_SystemError,
+            "Negative size passed to PyBytes_FromStringAndSize");
+        return NULL;
+    }
 
     new = PyObject_New(PyBytesObject, &PyBytes_Type);
     if (new == NULL)