diff -r 9abb316f1593 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Sun Oct 02 13:08:47 2016 +0300
+++ b/Lib/test/test_unicode.py	Sun Oct 02 18:53:16 2016 +0800
@@ -2594,6 +2594,23 @@
         self.assertEqual(size, nchar)
         self.assertEqual(wchar, nonbmp + '\0')
 
+    # Test PyUnicode_AsUCS4()
+    @support.cpython_only
+    def test_asucs4(self):
+        from _testcapi import unicode_asucs4
+        for s in ['abc', '\xa1\xa2', '\u4f60\u597d', 'a\ud800b\udfffc',
+                  '\ud834\udd1e', 'a\U0001f600']:
+            l = len(s)
+            self.assertEqual(unicode_asucs4(s, l, 1), s+'\0')
+            self.assertEqual(unicode_asucs4(s, l, 0), s+'\uffff')
+            self.assertEqual(unicode_asucs4(s, l+1, 1), s+'\0\uffff')
+            self.assertEqual(unicode_asucs4(s, l+1, 0), s+'\0\uffff')
+            self.assertRaises(ValueError, unicode_asucs4, s, l-1, 1)
+            self.assertRaises(ValueError, unicode_asucs4, s, l-2, 0)
+            s = '\0'.join([s, s])
+            self.assertEqual(unicode_asucs4(s, len(s), 1), s+'\0')
+            self.assertEqual(unicode_asucs4(s, len(s), 0), s+'\uffff')
+
     def test_subclass_add(self):
         class S(str):
             def __add__(self, o):
diff -r 9abb316f1593 Modules/_testcapimodule.c
--- a/Modules/_testcapimodule.c	Sun Oct 02 13:08:47 2016 +0300
+++ b/Modules/_testcapimodule.c	Sun Oct 02 18:53:16 2016 +0800
@@ -1829,6 +1829,36 @@
 }
 
 static PyObject *
+unicode_asucs4(PyObject *self, PyObject *args)
+{
+    PyObject *unicode, *result;
+    Py_UCS4 *buffer;
+    int copy_null;
+    Py_ssize_t str_len, buf_len;
+
+    if (!PyArg_ParseTuple(args, "Unp:unicode_asucs4", &unicode, &str_len, &copy_null)) {
+        return NULL;
+    }
+
+    buf_len = str_len + 1;
+    buffer = PyMem_NEW(Py_UCS4, buf_len);
+    if (buffer == NULL) {
+        return PyErr_NoMemory();
+    }
+    memset(buffer, 0, sizeof(Py_UCS4)*buf_len);
+    buffer[str_len] = 0xffffU;
+
+    if (!PyUnicode_AsUCS4(unicode, buffer, buf_len, copy_null)) {
+        PyMem_FREE(buffer);
+        return NULL;
+    }
+
+    result = PyUnicode_FromKindAndData(PyUnicode_4BYTE_KIND, buffer, buf_len);
+    PyMem_FREE(buffer);
+    return result;
+}
+
+static PyObject *
 unicode_encodedecimal(PyObject *self, PyObject *args)
 {
     Py_UNICODE *unicode;
@@ -4030,6 +4060,7 @@
     {"test_widechar",           (PyCFunction)test_widechar,      METH_NOARGS},
     {"unicode_aswidechar",      unicode_aswidechar,              METH_VARARGS},
     {"unicode_aswidecharstring",unicode_aswidecharstring,        METH_VARARGS},
+    {"unicode_asucs4",          unicode_asucs4,                  METH_VARARGS},
     {"unicode_encodedecimal",   unicode_encodedecimal,           METH_VARARGS},
     {"unicode_transformdecimaltoascii", unicode_transformdecimaltoascii, METH_VARARGS},
     {"unicode_legacy_string",   unicode_legacy_string,           METH_VARARGS},
diff -r 9abb316f1593 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Sun Oct 02 13:08:47 2016 +0300
+++ b/Objects/unicodeobject.c	Sun Oct 02 18:53:16 2016 +0800
@@ -2437,7 +2437,7 @@
     }
     else {
         if (targetsize < targetlen) {
-            PyErr_Format(PyExc_SystemError,
+            PyErr_Format(PyExc_ValueError,
                          "string is longer than the buffer");
             if (copy_null && 0 < targetsize)
                 target[0] = 0;