diff --git a/src/detail/ca-state.hpp b/src/detail/ca-state.hpp
index 23ad6db..994d185 100644
--- a/src/detail/ca-state.hpp
+++ b/src/detail/ca-state.hpp
@@ -135,7 +135,7 @@
   /**
    * @brief The challenge state.
    */
-  boost::optional<ChallengeState> m_challengeState;
+  optional<ChallengeState> m_challengeState;
 };
 
 std::ostream&