diff --git a/gateway/platforms/matrix.py b/gateway/platforms/matrix.py
index 7033e6171a..15589d9910 100644
--- a/gateway/platforms/matrix.py
+++ b/gateway/platforms/matrix.py
@@ -542,9 +542,9 @@ class MatrixAdapter(BasePlatformAdapter):
                 # no megolm sessions ever land. Setting _device_id here
                 # (in-memory; the on-disk row may not exist yet) makes
                 # the first put_account write the correct value.
+                # DeviceID is a NewType(str) so plain str works at runtime.
                 if client.device_id:
-                    from mautrix.types import DeviceID as _DeviceID
-                    await crypto_store.put_device_id(_DeviceID(client.device_id))
+                    await crypto_store.put_device_id(client.device_id)
 
                 crypto_state = _CryptoStateStore(state_store, self._joined_rooms)
                 olm = OlmMachine(client, crypto_store, crypto_state)
diff --git a/tests/gateway/test_matrix.py b/tests/gateway/test_matrix.py
index a088ad9ba8..50a8a66756 100644
--- a/tests/gateway/test_matrix.py
+++ b/tests/gateway/test_matrix.py
@@ -197,10 +197,14 @@ def _make_fake_mautrix():
             self.account_id = account_id
             self.pickle_key = pickle_key
             self.db = db
+            self._device_id = ""
 
         async def open(self):
             pass
 
+        async def put_device_id(self, device_id):
+            self._device_id = device_id
+
     mautrix_crypto_store_asyncpg.PgCryptoStore = PgCryptoStore
 
     # --- mautrix.util ---