diff --git a/datasette/default_magic_parameters.py b/datasette/default_magic_parameters.py
index 239637e7..7a092f57 100644
--- a/datasette/default_magic_parameters.py
+++ b/datasette/default_magic_parameters.py
@@ -13,7 +13,7 @@ def header(key, request):
 
 def actor(key, request):
     if request.actor is None:
-        raise KeyError
+        raise KeyError(key)
     return request.actor[key]
 
 
diff --git a/tests/test_stored_queries.py b/tests/test_stored_queries.py
index e0b0e6c6..4e615241 100644
--- a/tests/test_stored_queries.py
+++ b/tests/test_stored_queries.py
@@ -5,6 +5,9 @@ import pytest
 from asgiref.sync import async_to_sync
 from bs4 import BeautifulSoup as Soup
 
+from datasette.utils.asgi import Request
+from datasette.views.database import MagicParameters
+
 from .fixtures import make_app_client
 
 
@@ -343,6 +346,27 @@ def magic_parameters_client():
         yield client
 
 
+@pytest.mark.parametrize(
+    "actor,key",
+    [
+        (None, "id"),
+        (None, "name"),
+        ({}, "id"),
+        ({"id": "root"}, "name"),
+    ],
+)
+@pytest.mark.asyncio
+async def test_magic_actor_missing_key(bare_ds, actor, key):
+    request = Request.fake("/")
+    request.scope["actor"] = actor
+    parameters = MagicParameters(f"select :_actor_{key}", {}, request, bare_ds)
+
+    with pytest.raises(KeyError) as exc_info:
+        await parameters.execute_params()
+
+    assert exc_info.value.args == (key,)
+
+
 @pytest.mark.parametrize(
     "magic_parameter,expected_re",
     [