diff --git a/extensions/openpower-pels/mru.hpp b/extensions/openpower-pels/mru.hpp
index 8ad3e3c..e2293f1 100644
--- a/extensions/openpower-pels/mru.hpp
+++ b/extensions/openpower-pels/mru.hpp
@@ -61,7 +61,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    void flatten(Stream& pel);
+    void flatten(Stream& pel) const;
 
     /**
      * @brief Returns the size of this structure when flattened into a PEL