diff --git a/CHANGES.md b/CHANGES.md
index 5672caec54..a50d73fda9 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -8,6 +8,17 @@
 
 - Drop support for unstable field names from the long-accepted [MSC2732](https://github.com/matrix-org/matrix-spec-proposals/pull/2732) (Olm fallback keys) proposal. This change allows unit tests to pass following the security patch above. ([\#18996](https://github.com/element-hq/synapse/issues/18996))
 
+
+
+# Synapse 1.138.4 (2025-10-07)
+
+## Bugfixes
+
+- Fix a bug introduced in 1.138.3 where a client could receive an Internal Server Error if they set `device_keys: null` in the request to [`POST /_matrix/client/v3/keys/upload`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3keysupload). ([\#19023](https://github.com/element-hq/synapse/issues/19023))
+
+
+
+
 # Synapse 1.138.3 (2025-10-07)
 
 ## Security Fixes
diff --git a/debian/changelog b/debian/changelog
index a666727c87..8a90f45d5e 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -4,6 +4,12 @@ matrix-synapse-py3 (1.139.1) stable; urgency=medium
 
  -- Synapse Packaging team <packages@matrix.org>  Tue, 07 Oct 2025 11:46:51 +0100
 
+matrix-synapse-py3 (1.138.4) stable; urgency=medium
+
+  * New Synapse release 1.138.4.
+
+ -- Synapse Packaging team <packages@matrix.org>  Tue, 07 Oct 2025 16:28:38 +0100
+
 matrix-synapse-py3 (1.138.3) stable; urgency=medium
 
   * New Synapse release 1.138.3.
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index 9d22e22b72..55922b97d4 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -270,7 +270,7 @@ class KeyUploadServlet(RestServlet):
                 400, "To upload keys, you must pass device_id when authenticating"
             )
 
-        if "device_keys" in body:
+        if "device_keys" in body and isinstance(body["device_keys"], dict):
             # Validate the provided `user_id` and `device_id` fields in
             # `device_keys` match that of the requesting user. We can't do
             # this directly in the pydantic model as we don't have access
@@ -278,13 +278,13 @@ class KeyUploadServlet(RestServlet):
             #
             # TODO: We could use ValidationInfo when we switch to Pydantic v2.
             # https://docs.pydantic.dev/latest/concepts/validators/#validation-info
-            if body["device_keys"]["user_id"] != user_id:
+            if body["device_keys"].get("user_id") != user_id:
                 raise SynapseError(
                     code=HTTPStatus.BAD_REQUEST,
                     errcode=Codes.BAD_JSON,
                     msg="Provided `user_id` in `device_keys` does not match that of the authenticated user",
                 )
-            if body["device_keys"]["device_id"] != device_id:
+            if body["device_keys"].get("device_id") != device_id:
                 raise SynapseError(
                     code=HTTPStatus.BAD_REQUEST,
                     errcode=Codes.BAD_JSON,
diff --git a/tests/rest/client/test_keys.py b/tests/rest/client/test_keys.py
index ef3aef5dc8..817edfb8d3 100644
--- a/tests/rest/client/test_keys.py
+++ b/tests/rest/client/test_keys.py
@@ -160,6 +160,26 @@ class KeyUploadTestCase(unittest.HomeserverTestCase):
             channel.result,
         )
 
+    def test_upload_keys_succeeds_when_fields_are_explicitly_set_to_null(self) -> None:
+        """
+        This is a regression test for https://github.com/element-hq/synapse/pull/19023.
+        """
+        device_id = "DEVICE_ID"
+        self.register_user("alice", "wonderland")
+        alice_token = self.login("alice", "wonderland", device_id=device_id)
+
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/upload",
+            {
+                "device_keys": None,
+                "one_time_keys": None,
+                "fallback_keys": None,
+            },
+            alice_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
 
 class KeyQueryTestCase(unittest.HomeserverTestCase):
     servlets = [