diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index cb0e20b..657b200 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -105,8 +105,11 @@
 PyObject *
 PyString_FromString(const char *str)
 {
-	register size_t size = strlen(str);
+	register size_t size;
 	register PyStringObject *op;
+
+	assert(str != NULL);
+	size = strlen(str);
 	if (size > INT_MAX) {
 		PyErr_SetString(PyExc_OverflowError,
 			"string is too long for a Python string");