diff --git a/django/contrib/auth/backends.py b/django/contrib/auth/backends.py
index cb79291c17..006c88d5de 100644
--- a/django/contrib/auth/backends.py
+++ b/django/contrib/auth/backends.py
@@ -64,8 +64,8 @@ class ModelBackend(object):
         return False
 
     def get_user(self, user_id):
+        UserModel = get_user_model()
         try:
-            UserModel = get_user_model()
             return UserModel._default_manager.get(pk=user_id)
         except UserModel.DoesNotExist:
             return None
diff --git a/django/contrib/auth/tests/test_auth_backends.py b/django/contrib/auth/tests/test_auth_backends.py
index 8bacaea245..be53aa6a00 100644
--- a/django/contrib/auth/tests/test_auth_backends.py
+++ b/django/contrib/auth/tests/test_auth_backends.py
@@ -480,3 +480,27 @@ class ChangedBackendSettingsTest(TestCase):
             # anonymous as the backend is not longer available.
             self.assertIsNotNone(user)
             self.assertTrue(user.is_anonymous())
+
+
+@skipIfCustomUser
+class ImproperlyConfiguredUserModelTest(TestCase):
+    """
+    Tests that an exception from within get_user_model is propagated and doesn't
+    raise an UnboundLocalError.
+
+    Regression test for ticket #21439
+    """
+    def setUp(self):
+        self.user1 = User.objects.create_user('test', 'test@example.com', 'test')
+        self.client.login(
+            username='test',
+            password='test'
+        )
+
+    @override_settings(AUTH_USER_MODEL='thismodel.doesntexist')
+    def test_does_not_shadow_exception(self):
+        # Prepare a request object
+        request = HttpRequest()
+        request.session = self.client.session
+
+        self.assertRaises(ImproperlyConfigured, get_user, request)