diff --git a/src/detail/challenge-encoder.hpp b/src/detail/challenge-encoder.hpp
index b47938e..541ad60 100644
--- a/src/detail/challenge-encoder.hpp
+++ b/src/detail/challenge-encoder.hpp
@@ -34,7 +34,7 @@
   encodeDataContent(const ca::RequestState& request);
 
   static void
-  decodeDataContent(const Block& data, RequesterState& state);
+  decodeDataContent(const Block& data, requester::RequesterState& state);
 };
 
 } // namespace ndncert