diff --git a/src/jwkest/jws.py b/src/jwkest/jws.py
index f17e6f6..87e3038 100644
--- a/src/jwkest/jws.py
+++ b/src/jwkest/jws.py
@@ -696,7 +696,7 @@ def is_jws(self, jws):
 
     def _is_json_serialized_jws(self, json_jws):
         json_ser_keys = set(["payload", "signatures"])
-        flattened_json_ser_keys = set[("payload", "signature")]
+        flattened_json_ser_keys = set(["payload", "signature"])
         if not json_ser_keys.issubset(
                 json_jws.keys()) and not flattened_json_ser_keys.issubset(
                 json_jws.keys()):