diff --git a/django/contrib/auth/context_processors.py b/django/contrib/auth/context_processors.py
index 3ffab01e94..ea9f95c4f0 100644
--- a/django/contrib/auth/context_processors.py
+++ b/django/contrib/auth/context_processors.py
@@ -11,6 +11,11 @@ class PermLookupDict(object):
     def __getitem__(self, perm_name):
         return self.user.has_perm("%s.%s" % (self.module_name, perm_name))
 
+    def __iter__(self):
+        # To fix 'item in perms.someapp' and __getitem__ iteraction we need to
+        # define __iter__. See #18979 for details.
+        raise TypeError("PermLookupDict is not iterable.")
+
     def __nonzero__(self):
         return self.user.has_module_perms(self.module_name)
 
diff --git a/django/contrib/auth/tests/context_processors.py b/django/contrib/auth/tests/context_processors.py
index dcd50fc6cc..55cbdd413e 100644
--- a/django/contrib/auth/tests/context_processors.py
+++ b/django/contrib/auth/tests/context_processors.py
@@ -2,12 +2,56 @@ import os
 
 from django.conf import global_settings
 from django.contrib.auth import authenticate
+from django.contrib.auth.context_processors import PermWrapper, PermLookupDict
 from django.db.models import Q
 from django.template import context
 from django.test import TestCase
 from django.test.utils import override_settings
 
 
+class MockUser(object):
+    def has_module_perm(self, perm):
+        if perm == 'mockapp.someapp':
+            return True
+        return False
+
+    def has_perm(self, perm):
+        if perm == 'someperm':
+            return True
+        return False
+
+
+class PermWrapperTests(TestCase):
+    """
+    Test some details of the PermWrapper implementation.
+    """
+    class EQLimiterObject(object):
+        """
+        This object makes sure __eq__ will not be called endlessly.
+        """
+        def __init__(self):
+            self.eq_calls = 0
+
+        def __eq__(self, other):
+            if self.eq_calls > 0:
+                return True
+            self.eq_calls += 1
+            return False
+
+    def test_permwrapper_in(self):
+        """
+        Test that 'something' in PermWrapper doesn't end up in endless loop.
+        """
+        perms = PermWrapper(MockUser())
+        with self.assertRaises(TypeError):
+            self.EQLimiterObject() in perms
+
+    def test_permlookupdict_in(self):
+        pldict = PermLookupDict(MockUser(), 'mockapp')
+        with self.assertRaises(TypeError):
+            self.EQLimiterObject() in pldict
+
+
 class AuthContextProcessorTests(TestCase):
     """
     Tests for the ``django.contrib.auth.context_processors.auth`` processor