diff -r f2f5d1c928eb Lib/test/test_bytes.py
--- a/Lib/test/test_bytes.py	Sun Jun 28 11:15:13 2015 -0400
+++ b/Lib/test/test_bytes.py	Sun Jun 28 22:26:20 2015 +0300
@@ -1098,9 +1098,27 @@ class ByteArrayTest(BaseBytesTest, unitt
         for i in range(100):
             b += b"x"
             alloc = b.__alloc__()
-            self.assertTrue(alloc >= len(b))
+            self.assertGreater(alloc, len(b))  # including trailing null byte
             if alloc not in seq:
                 seq.append(alloc)
+        print(seq)
+
+    def test_init_alloc(self):
+        b = bytearray()
+        def g():
+            for i in range(1, 100):
+                yield i
+                a = list(b)
+                self.assertEqual(a, list(range(1, len(a)+1)))
+                self.assertEqual(len(b), len(a))
+                self.assertLessEqual(len(b), i)
+                alloc = b.__alloc__()
+                self.assertGreater(alloc, len(b))  # including trailing null byte
+        b.__init__(g())
+        self.assertEqual(list(b), list(range(1, 100)))
+        self.assertEqual(len(b), 99)
+        alloc = b.__alloc__()
+        self.assertGreater(alloc, len(b))
 
     def test_extend(self):
         orig = b'hello'
diff -r f2f5d1c928eb Objects/bytearrayobject.c
--- a/Objects/bytearrayobject.c	Sun Jun 28 11:15:13 2015 -0400
+++ b/Objects/bytearrayobject.c	Sun Jun 28 22:26:20 2015 +0300
@@ -891,8 +891,10 @@ bytearray_init(PyByteArrayObject *self, 
             goto error;
 
         /* Append the byte */
-        if (Py_SIZE(self) < self->ob_alloc)
+        if (Py_SIZE(self) + 1 < self->ob_alloc) {
             Py_SIZE(self)++;
+            PyByteArray_AS_STRING(self)[Py_SIZE(self)] = '\0';
+        }
         else if (PyByteArray_Resize((PyObject *)self, Py_SIZE(self)+1) < 0)
             goto error;
         PyByteArray_AS_STRING(self)[Py_SIZE(self)-1] = value;