diff --git a/changelog.d/15929.feature b/changelog.d/15929.feature
index f0814ec15dd9..c3aaeae66ee7 100644
--- a/changelog.d/15929.feature
+++ b/changelog.d/15929.feature
@@ -1,3 +1 @@
-Implement [MSC3814](https://github.com/matrix-org/matrix-spec-proposals/pull/3814),
-dehydrated devices v2/shrivelled sessions and move [MSC2697](https://github.com/matrix-org/matrix-spec-proposals/pull/2697)
-behind a config flag. Contributed by Nico from Famedly and H-Shay.
+Implement [MSC3814](https://github.com/matrix-org/matrix-spec-proposals/pull/3814), dehydrated devices v2/shrivelled sessions and move [MSC2697](https://github.com/matrix-org/matrix-spec-proposals/pull/2697) behind a config flag. Contributed by Nico from Famedly and H-Shay.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index e8cd1e68c8c1..1695ed8ca336 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -259,6 +259,15 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         # once.
         self.msc3814_enabled: bool = experimental.get("msc3814_enabled", False)
 
+        if self.msc2697_enabled and self.msc3814_enabled:
+            raise ConfigError(
+                "MSC2697 and MSC3814 should not both be enabled.",
+                (
+                    "experimental_features",
+                    "msc3814_enabled",
+                ),
+            )
+
         # MSC3244 (room version capabilities)
         self.msc3244_enabled: bool = experimental.get("msc3244_enabled", True)
 
diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 621f4c4449c1..a541d9703cc2 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -331,7 +331,15 @@ async def get_events_for_dehydrated_device(
         # only allow fetching messages for the dehydrated device id currently associated
         # with the user
         dehydrated_device = await self.device_handler.get_dehydrated_device(user_id)
-        if dehydrated_device is None or device_id != dehydrated_device[0]:
+        if dehydrated_device is None:
+            raise SynapseError(
+                HTTPStatus.FORBIDDEN,
+                "You may only fetch messages for your dehydrated device",
+                Codes.FORBIDDEN,
+            )
+
+        dehydrated_device_id = dehydrated_device[0]
+        if device_id != dehydrated_device_id:
             raise SynapseError(
                 HTTPStatus.FORBIDDEN,
                 "You may only fetch messages for your dehydrated device",
diff --git a/synapse/rest/client/devices.py b/synapse/rest/client/devices.py
index 3223764464a4..89776f639263 100644
--- a/synapse/rest/client/devices.py
+++ b/synapse/rest/client/devices.py
@@ -231,7 +231,7 @@ class Config:
 class DehydratedDeviceServlet(RestServlet):
     """Retrieve or store a dehydrated device.
 
-    Implements both MSC2697 and MSC3814.
+    Implements either MSC2697 and MSC3814.
 
     GET /org.matrix.msc2697.v2/dehydrated_device