diff --git a/ccnx/sync-app-socket.h b/ccnx/sync-app-socket.h
index 07c0aa0..bb533a9 100644
--- a/ccnx/sync-app-socket.h
+++ b/ccnx/sync-app-socket.h
@@ -82,7 +82,7 @@
   void fetchString(const std::string &prefix, const SeqNo &seq, CcnxWrapper::StringDataCallback callback, int retry = 0);
   void fetchRaw(const std::string &prefix, const SeqNo &seq, CcnxWrapper::RawDataCallback callback, int retry = 0);
 
-  void passCallback(std::vector<MissingDataInfo> &v) {m_newDataCallback(v, this);}
+  void passCallback(const std::vector<MissingDataInfo> &v) {m_newDataCallback(v, this);}
 
 private:
   uint32_t