diff --git a/synapse/rest/client/v2_alpha/keys.py b/synapse/rest/client/v2_alpha/keys.py
index f99b53530..6a3cfe84f 100644
--- a/synapse/rest/client/v2_alpha/keys.py
+++ b/synapse/rest/client/v2_alpha/keys.py
@@ -193,7 +193,7 @@ class KeyChangesServlet(RestServlet):
         )
 
         defer.returnValue((200, {
-            "changed": changed
+            "changed": list(changed),
         }))