diff --git a/extensions/openpower-pels/callouts.hpp b/extensions/openpower-pels/callouts.hpp
index 430a5ae..9226a1b 100644
--- a/extensions/openpower-pels/callouts.hpp
+++ b/extensions/openpower-pels/callouts.hpp
@@ -44,7 +44,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    void flatten(Stream& pel);
+    void flatten(Stream& pel) const;
 
     /**
      * @brief Returns the size of this object when flattened into a PEL