diff -r c852c7d8d681 Lib/test/test_bytes.py
--- a/Lib/test/test_bytes.py	Sun Nov 29 13:14:00 2015 +0200
+++ b/Lib/test/test_bytes.py	Sun Nov 29 22:23:41 2015 +0200
@@ -793,6 +793,11 @@ class BytesTest(BaseBytesTest, unittest.
             def __index__(self):
                 return 42
         self.assertEqual(bytes(A()), b'a')
+        # Issue #XXX
+        class A(str):
+            def __bytes__(self):
+                return b'abc'
+        self.assertEqual(bytes(A()), b'abc')
         # Issue #24731
         class A:
             def __bytes__(self):
diff -r c852c7d8d681 Objects/bytesobject.c
--- a/Objects/bytesobject.c	Sun Nov 29 13:14:00 2015 +0200
+++ b/Objects/bytesobject.c	Sun Nov 29 22:23:41 2015 +0200
@@ -3320,11 +3320,11 @@ bytes_new(PyTypeObject *type, PyObject *
         return PyBytes_FromStringAndSize(NULL, 0);
     }
 
-    if (PyUnicode_Check(x)) {
+    if (encoding != NULL) {
         /* Encode via the codec registry */
-        if (encoding == NULL) {
+        if (!PyUnicode_Check(x)) {
             PyErr_SetString(PyExc_TypeError,
-                            "string argument without an encoding");
+                            "encoding without a string argument");
             return NULL;
         }
         new = PyUnicode_AsEncodedString(x, encoding, errors);
@@ -3334,10 +3334,11 @@ bytes_new(PyTypeObject *type, PyObject *
         return new;
     }
 
-    /* If it's not unicode, there can't be encoding or errors */
-    if (encoding != NULL || errors != NULL) {
+    if (errors != NULL) {
         PyErr_SetString(PyExc_TypeError,
-            "encoding or errors without a string argument");
+                        PyUnicode_Check(x) ?
+                        "string argument without an encoding" :
+                        "errors without a string argument");
         return NULL;
     }
 
@@ -3362,6 +3363,11 @@ bytes_new(PyTypeObject *type, PyObject *
     else if (PyErr_Occurred())
         return NULL;
 
+    if (PyUnicode_Check(x)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "string argument without an encoding");
+        return NULL;
+    }
     /* Is it an integer? */
     size = PyNumber_AsSsize_t(x, PyExc_OverflowError);
     if (size == -1 && PyErr_Occurred()) {