Refs #8467 -- Added test for RelatedManager.add()/remove() with an invalid type.

tests/many_to_many/tests.py

@@ -104,6 +104,12 @@ class ManyToManyTests(TestCase):
         a5.authors.remove(user_2.username)
         self.assertQuerysetEqual(a5.authors.all(), [])
 
+    def test_add_remove_invalid_type(self):
+        msg = "Field 'id' expected a number but got 'invalid'."
+        for method in ['add', 'remove']:
+            with self.subTest(method), self.assertRaisesMessage(ValueError, msg):
+                getattr(self.a1.publications, method)('invalid')
+
     def test_reverse_add(self):
         # Adding via the 'other' end of an m2m
         a5 = Article(headline='NASA finds intelligent life on Mars')