diff --git a/extensions/openpower-pels/user_header.hpp b/extensions/openpower-pels/user_header.hpp
index b382a84..a1873f5 100644
--- a/extensions/openpower-pels/user_header.hpp
+++ b/extensions/openpower-pels/user_header.hpp
@@ -59,7 +59,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    void flatten(Stream& stream) override;
+    void flatten(Stream& stream) const override;
 
     /**
      * @brief Returns the subsystem field.