diff --git a/changelog.d/8975.bugfix b/changelog.d/8975.bugfix
new file mode 100644
index 000000000..75049b8e1
--- /dev/null
+++ b/changelog.d/8975.bugfix
@@ -0,0 +1 @@
+Add validation to the `sendToDevice` API to raise a missing parameters error instead of a 500 error.
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index bc4f43639..a3dee14ed 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -17,7 +17,7 @@ import logging
 from typing import Tuple
 
 from synapse.http import servlet
-from synapse.http.servlet import parse_json_object_from_request
+from synapse.http.servlet import assert_params_in_dict, parse_json_object_from_request
 from synapse.logging.opentracing import set_tag, trace
 from synapse.rest.client.transactions import HttpTransactionCache
 
@@ -54,6 +54,7 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         content = parse_json_object_from_request(request)
+        assert_params_in_dict(content, ("messages",))
 
         sender_user_id = requester.user.to_string()