diff --git a/nsync/sync-socket.h b/nsync/sync-socket.h
index dce6fde..35b59c3 100644
--- a/nsync/sync-socket.h
+++ b/nsync/sync-socket.h
@@ -106,7 +106,7 @@
   }
 
   void
-  onData(const ndn::Interest& interest, ndn::Data& data,
+  onData(const ndn::Interest& interest, const ndn::Data& data,
          const ndn::OnDataValidated& onValidated,
          const ndn::OnDataValidationFailed& onValidationFailed);