diff --git a/src/challenge/challenge-email.hpp b/src/challenge/challenge-email.hpp
index c005b7e..e2c7267 100644
--- a/src/challenge/challenge-email.hpp
+++ b/src/challenge/challenge-email.hpp
@@ -65,7 +65,7 @@
 
   Block
   genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                         std::multimap<std::string, std::string>&& params) override;
+                         const std::multimap<std::string, std::string>& params) override;
 
   // challenge status
   static const std::string NEED_CODE;