Make setting 'txn.user = None' raise ValueError.

transaction/_transaction.py

@@ -136,10 +136,9 @@ class Transaction(object):
 
     @user.setter
     def user(self, v):
-        if v is not None:
-            self._user = text_or_warn(v)
-        else:
-            self._user = u""
+        if v is None:
+            raise ValueError("user must not be None")
+        self._user = text_or_warn(v)
 
     @property
     def description(self):

transaction/tests/test__transaction.py

@@ -1063,8 +1063,9 @@ class TransactionTests(unittest.TestCase):
     def test_user_w_none(self):
         txn = self._makeOne()
         txn.user = b'phreddy'
-        txn.user = None  # resets to empty text
-        self.assertEqual(txn.user, u'')
+        with self.assertRaises(ValueError):
+            txn.user = None  # resets to empty text
+        self.assertEqual(txn.user, b'phreddy')
 
     def _test_user_non_text(self, user, path, expect, both=False):
         txn = self._makeOne()