diff -r 1be4d7388279 Lib/test/test_json/test_separators.py
--- a/Lib/test/test_json/test_separators.py	Sun Jul 26 06:50:51 2015 +1200
+++ b/Lib/test/test_json/test_separators.py	Sun Jul 26 08:26:13 2015 +0300
@@ -39,6 +39,12 @@ class TestSeparators:
         self.assertEqual(h2, h)
         self.assertEqual(d2, expect)
 
+    def test_illegal_separators(self):
+        h = {1: 2, 3: 4}
+        self.assertRaises(TypeError, self.dumps, h, separators=(b', ', ': '))
+        self.assertRaises(TypeError, self.dumps, h, separators=(', ', b': '))
+        self.assertRaises(TypeError, self.dumps, h, separators=(b', ', b': '))
+
 
 class TestPySeparators(TestSeparators, PyTest): pass
 class TestCSeparators(TestSeparators, CTest): pass
diff -r 1be4d7388279 Modules/_json.c
--- a/Modules/_json.c	Sun Jul 26 06:50:51 2015 +1200
+++ b/Modules/_json.c	Sun Jul 26 08:26:13 2015 +0300
@@ -1344,11 +1344,19 @@ encoder_init(PyObject *self, PyObject *a
     assert(PyEncoder_Check(self));
     s = (PyEncoderObject *)self;
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "OOOOOOOOp:make_encoder", kwlist,
-        &markers, &defaultfn, &encoder, &indent, &key_separator, &item_separator,
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "OOOOUUOOp:make_encoder", kwlist,
+        &markers, &defaultfn, &encoder, &indent,
+        &key_separator, &item_separator,
         &sort_keys, &skipkeys, &allow_nan))
         return -1;
 
+    if (markers != Py_None && !PyDict_Check(markers)) {
+        PyErr_Format(PyExc_TypeError,
+                     "make_encoder() argument 1 must be dict or None, "
+                     "not %.200s", Py_TYPE(markers)->tp_name);
+        return -1;
+    }
+
     s->markers = markers;
     s->defaultfn = defaultfn;
     s->encoder = encoder;