diff --git a/src/firebase_functions/pubsub_fn.py b/src/firebase_functions/pubsub_fn.py
index 8733822..bce81f7 100644
--- a/src/firebase_functions/pubsub_fn.py
+++ b/src/firebase_functions/pubsub_fn.py
@@ -87,9 +87,9 @@ class MessagePublishedData(_typing.Generic[T]):
     Google Cloud Pub/Sub message.
     """
 
-    subscription: str
+    subscription: str | None
     """
-    A subscription resource.
+    A subscription resource. ``None`` when the request had no CloudEvent headers.
     """
 
 
@@ -107,6 +107,12 @@ def _message_handler(
     data = event_dict["data"]
     message_dict = data["message"]
 
+    # When a request has no CloudEvent headers, functions-framework builds the
+    # message itself with camelCase keys only, an "@type" key and no subscription.
+    message_dict.setdefault("message_id", message_dict.get("messageId"))
+    message_dict.setdefault("publish_time", message_dict.get("publishTime"))
+    message_dict.pop("@type", None)
+
     # if no microseconds are present, we should set them to 0 to prevent parsing from failing
     if "." not in event_dict["time"]:
         event_dict["time"] = event_dict["time"].replace("Z", ".000000Z")
@@ -147,7 +153,7 @@ def _message_handler(
             **message_dict,
             ordering_key=ordering_key,
         ),
-        subscription=data["subscription"],
+        subscription=data.get("subscription"),
     )
 
     event_dict["data"] = message
diff --git a/tests/test_pubsub_fn.py b/tests/test_pubsub_fn.py
index d1c4c56..7feba0e 100644
--- a/tests/test_pubsub_fn.py
+++ b/tests/test_pubsub_fn.py
@@ -18,6 +18,8 @@
 from unittest.mock import MagicMock
 
 from cloudevents.http import CloudEvent as _CloudEvent
+from flask import Flask, request
+from functions_framework import event_conversion
 
 from firebase_functions import core
 from firebase_functions.pubsub_fn import (
@@ -182,3 +184,29 @@ def test_publish_time_is_timezone_aware_datetime(self):
             _dt.datetime(2023, 3, 11, 13, 25, 37, 403000, tzinfo=_dt.timezone.utc),
         )
         self.assertEqual(message.publish_time.utcoffset(), _dt.timedelta(0))
+
+    def test_push_body_without_cloudevent_headers_reaches_handler(self):
+        push_body = {
+            "message": {
+                "attributes": {"key": "value"},
+                "data": "eyJ0ZXN0IjogInZhbHVlIn0=",
+                "messageId": "message-id-123",
+                "publishTime": "2023-03-11T13:25:37.403Z",
+            },
+            "subscription": "projects/p/subscriptions/s",
+        }
+        with Flask(__name__).test_request_context("/", method="POST", json=push_body):
+            raw = event_conversion.background_event_to_cloud_event(request)
+
+        func = MagicMock()
+        _message_handler(func, raw)
+
+        data = func.call_args.args[0].data
+        self.assertEqual(data.message.message_id, "message-id-123")
+        self.assertEqual(
+            data.message.publish_time,
+            _dt.datetime(2023, 3, 11, 13, 25, 37, 403000, tzinfo=_dt.timezone.utc),
+        )
+        self.assertEqual(data.message.attributes, {"key": "value"})
+        self.assertEqual(data.message.json, {"test": "value"})
+        self.assertIsNone(data.subscription)