diff --git a/activitypub.py b/activitypub.py
index 2a0b75d..e94dcc0 100644
--- a/activitypub.py
+++ b/activitypub.py
@@ -527,8 +527,8 @@ def signed_request(fn, url, data=None, headers=None, **kwargs):
 def postprocess_as2(activity, orig_obj=None, wrap=True):
     """Prepare an AS2 object to be served or sent via ActivityPub.
 
-    ``g.user`` is required. Populates it into the ``actor.id`` and ``publicKey``
-    fields.
+    ``g.user`` is required (in ``postprocess_as2_actor``). It's populated it
+    into the ``actor.id`` and ``publicKey`` fields.
 
     Args:
       activity (dict): AS2 object or activity
diff --git a/convert.py b/convert.py
index 4379196..4a154a3 100644
--- a/convert.py
+++ b/convert.py
@@ -58,13 +58,6 @@ def convert(dest, _, src=None):
 
     logger.info(f'Converting from {src_cls.LABEL} to {dest}: {id}')
 
-    # require g.user for AP since postprocess_as2 currently needs it. ugh
-    if dest_cls == ActivityPub:
-        domain = util.domain_from_link(id, minimize=False)
-        g.user = Web.get_by_id(domain)
-        if not g.user:
-            error(f'No web user found for {domain}')
-
     # load, and maybe fetch. if it's a post/update, redirect to inner object.
     obj = src_cls.load(id)
     if not obj:
@@ -86,6 +79,22 @@ def convert(dest, _, src=None):
     if obj.deleted or type == 'delete':
         return '', 410
 
+    # load g.user for AP since postprocess_as2 currently needs it. ugh.
+    if dest_cls == ActivityPub:
+        actor_id = as1.get_owner(obj.as1)
+        if not actor_id and src_cls == Web:
+            actor_id = util.domain_from_link(id, minimize=False)
+        if not actor_id:
+            error(f"Couldn't determine actor id for {obj.as1}")
+
+        user_key = src_cls.key_for(actor_id)
+        if not user_key:
+            error(f"Couldn't determine {src_cls.LABEL} key for {actor_id}")
+
+        g.user = user_key.get()
+        if not g.user:
+            error(f'No {src_cls.LABEL} user found for {actor_id}')
+
     # convert and serve
     return dest_cls.convert(obj), {'Content-Type': dest_cls.CONTENT_TYPE}
 
diff --git a/tests/test_convert.py b/tests/test_convert.py
index bd5f047..dd845af 100644
--- a/tests/test_convert.py
+++ b/tests/test_convert.py
@@ -13,6 +13,7 @@ from oauth_dropins.webutil.util import json_loads, parse_mf2
 from . import testutil
 from .testutil import Fake, OtherFake
 
+from activitypub import ActivityPub
 from common import CONTENT_TYPE_HTML
 
 COMMENT_AS2 = {
@@ -95,6 +96,24 @@ class ConvertTest(testutil.TestCase):
             'foo': 'bar',
         }, json_loads(resp.get_data()))
 
+    def test_fake_to_activitypub(self):
+        self.make_user('fake:alice', cls=Fake)
+        self.store_object(id='fake:post', our_as1={
+            'actor': 'fake:alice',
+            'foo': 'bar',
+        })
+        resp = self.client.get('/convert/ap/fake:post',
+                               base_url='https://fa.brid.gy/')
+        self.assertEqual(200, resp.status_code)
+        self.assertEqual(ActivityPub.CONTENT_TYPE, resp.content_type)
+        self.assertEqual({
+            '@context': 'https://www.w3.org/ns/activitystreams',
+            'id': 'https://fa.brid.gy/convert/ap/fake:post',
+            'actor': 'https://fa.brid.gy/ap/fake:alice',
+            'foo': 'bar',
+            'to': ['https://www.w3.org/ns/activitystreams#Public'],
+        }, json_loads(resp.get_data()))
+
     def test_activitypub_to_web_object(self):
         url = 'https://user.com/bar?baz=baj&biff'
         Object(id=url, our_as1=COMMENT).put()
@@ -274,7 +293,10 @@ A ☕ reply
         self.assertEqual(200, resp.status_code)
         self.assert_equals(COMMENT_AS2, resp.json, ignore=['to'])
 
-    def test_web_to_activitypub_no_user(self):
+    @patch('requests.get')
+    def test_web_to_activitypub_no_user(self, mock_get):
+        mock_get.return_value = requests_response(HTML)  # protocol inference
+
         resp = self.client.get(f'/convert/ap/http://nope.com/post',
                                base_url='https://web.brid.gy/')
         self.assertEqual(400, resp.status_code)
diff --git a/web.py b/web.py
index 59e58e3..9744cf1 100644
--- a/web.py
+++ b/web.py
@@ -245,14 +245,13 @@ class Web(User, Protocol):
 
         If id is a domain, uses it as is. If it's a home page URL or fed.brid.gy
         or web.brid.gy AP actor URL, extracts the domain and uses that.
-        Otherwise, raises AssertionError.
+        Otherwise, returns None.
 
         Args:
-          id: str
+          id (str)
 
-        Raises:
-          ValueError
-          AssertionError
+        Returns:
+        ndb.Key or None:
         """
         if not id:
             return None