diff -r 34e7b9879e60 Lib/test/test_format.py
--- a/Lib/test/test_format.py	Sun Dec 25 16:22:23 2016 +0200
+++ b/Lib/test/test_format.py	Mon Dec 26 12:30:36 2016 +0200
@@ -388,6 +388,13 @@ class FormatTest(unittest.TestCase):
             else:
                 raise TestFailed('"%*d"%(maxsize, -127) should fail')
 
+    def test_nul(self):
+        # test the null character
+        testcommon("a\0b", (), 'a\0b')
+        testcommon("a%cb", (0,), 'a\0b')
+        testformat("a%sb", ('c\0d',), 'ac\0db')
+        testcommon(b"a%sb", (b'c\0d',), b'ac\0db')
+
     def test_non_ascii(self):
         testformat("\u20ac=%f", (1.0,), "\u20ac=1.000000")
 
diff -r 34e7b9879e60 Objects/bytearrayobject.c
--- a/Objects/bytearrayobject.c	Sun Dec 25 16:22:23 2016 +0200
+++ b/Objects/bytearrayobject.c	Mon Dec 26 12:30:36 2016 +0200
@@ -283,13 +283,15 @@ bytearray_format(PyByteArrayObject *self
 {
     PyObject *bytes_in, *bytes_out, *res;
     char *bytestring;
+    Py_ssize_t bytesize;
 
     if (self == NULL || !PyByteArray_Check(self) || args == NULL) {
         PyErr_BadInternalCall();
         return NULL;
     }
     bytestring = PyByteArray_AS_STRING(self);
-    bytes_in = PyBytes_FromString(bytestring);
+    bytesize = PyByteArray_GET_SIZE(self);
+    bytes_in = PyBytes_FromStringAndSize(bytestring, bytesize);
     if (bytes_in == NULL)
         return NULL;
     bytes_out = _PyBytes_Format(bytes_in, args);