diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 75ad9f0..4337972 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -465,6 +465,14 @@
 PyObject *PyUnicode_FromStringAndSize(const char *u, Py_ssize_t size)
 {
     PyUnicodeObject *unicode;
+
+	assert(size <= 0);
+	if (size < 0) {
+		PyErr_SetString(PyExc_SystemError,
+		    "Negative size passed to PyUnicode_FromStringAndSize");
+		return NULL;
+	}
+
     /* If the Unicode data is known at construction time, we can apply
        some optimizations which share commonly used objects.
        Also, this means the input must be UTF-8, so fall back to the