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)
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++;
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