Index: Lib/test/pickletester.py
===================================================================
--- Lib/test/pickletester.py	(revision 66951)
+++ Lib/test/pickletester.py	(working copy)
@@ -996,6 +996,20 @@
         pickle.Pickler(f, -1)
         pickle.Pickler(f, protocol=-1)
 
+    def test_bad_init(self):
+        # Test issue3664 (pickle can segfault from a badly initialized Pickler).
+        from io import BytesIO
+        # Override initialization without calling __init__() of the superclass.
+        class BadPickler(pickle.Pickler):
+            def __init__(self): pass
+
+        class BadUnpickler(pickle.Unpickler):
+            def __init__(self): pass
+
+        self.assertRaises(pickle.PicklingError, BadPickler().dump, 0)
+        self.assertRaises(pickle.UnpicklingError, BadUnpickler().load)
+
+
 class AbstractPersistentPicklerTests(unittest.TestCase):
 
     # This class defines persistent_id() and persistent_load()
Index: Modules/_pickle.c
===================================================================
--- Modules/_pickle.c	(revision 66951)
+++ Modules/_pickle.c	(working copy)
@@ -421,6 +421,11 @@
 {
     PyObject *data, *result;
 
+    if (self->write_buf == NULL) {
+        PyErr_SetString(PyExc_SystemError, "invalid write buffer");
+        return -1;
+    }
+
     if (s == NULL) {
         if (!(self->buf_size))
             return 0;
@@ -2378,6 +2383,16 @@
 {
     PyObject *obj;
 
+    /* Check whether the Pickler was initialized correctly (issue3664).
+       Developers often forget to call __init__() in their subclasses, which
+       would trigger a segfault without this check. */
+    if (self->write == NULL) {
+        PyErr_Format(PicklingError, 
+                     "Pickler.__init__() was not called by %s.__init__()",
+                     Py_TYPE(self)->tp_name);
+        return NULL;
+    }
+
     if (!PyArg_ParseTuple(args, "O:dump", &obj))
         return NULL;