diff --git a/extensions/openpower-pels/callout.cpp b/extensions/openpower-pels/callout.cpp
index 9386862..95c6408 100644
--- a/extensions/openpower-pels/callout.cpp
+++ b/extensions/openpower-pels/callout.cpp
@@ -88,7 +88,7 @@
     return size;
 }
 
-void Callout::flatten(Stream& pel)
+void Callout::flatten(Stream& pel) const
 {
     pel << _size << _flags << _priority << _locationCodeSize;