diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 41521e6990..325c0c4a9f 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -354,8 +354,11 @@ def _exception_to_failure(e):
         }
 
     # include ConnectionRefused and other errors
+    #
+    # Note that some Exceptions (notably twisted's ResponseFailed etc) don't
+    # give a string for e.message, which simplejson then fails to serialize.
     return {
-        "status": 503, "message": e.message,
+        "status": 503, "message": str(e.message),
     }