diff -r 482ec940b413 Lib/test/test_socket.py
--- a/Lib/test/test_socket.py	Thu Sep 10 19:02:34 2015 -0400
+++ b/Lib/test/test_socket.py	Fri Sep 11 19:38:47 2015 +1000
@@ -1282,6 +1282,12 @@
             except socket.gaierror:
                 pass
 
+        # Issue 24684: str subclasses should not be able to trigger an assertion failure
+        class BadString(str):
+             def encode(self, enc):
+                 return -1
+        self.assertRaises(TypeError, socket.getaddrinfo, BadString(), 1)
+
     def test_getnameinfo(self):
         # only IP addresses are allowed
         self.assertRaises(OSError, socket.getnameinfo, ('mail.python.org',0), 0)
diff -r 482ec940b413 Modules/socketmodule.c
--- a/Modules/socketmodule.c	Thu Sep 10 19:02:34 2015 -0400
+++ b/Modules/socketmodule.c	Fri Sep 11 19:38:47 2015 +1000
@@ -5518,7 +5518,12 @@
         idna = _PyObject_CallMethodId(hobj, &PyId_encode, "s", "idna");
         if (!idna)
             return NULL;
-        assert(PyBytes_Check(idna));
+        if (!PyBytes_Check(idna)) {
+            Py_DECREF(idna);
+            PyErr_SetString(PyExc_TypeError,
+                            "getaddrinfo() argument 1 must be encodable string or None");
+            return NULL;
+        }
         hptr = PyBytes_AS_STRING(idna);
     } else if (PyBytes_Check(hobj)) {
         hptr = PyBytes_AsString(hobj);