diff --git a/extensions/openpower-pels/failing_mtms.cpp b/extensions/openpower-pels/failing_mtms.cpp
index 8b01ff7..d4cdf37 100644
--- a/extensions/openpower-pels/failing_mtms.cpp
+++ b/extensions/openpower-pels/failing_mtms.cpp
@@ -75,7 +75,7 @@
     _valid = (failed) ? false : true;
 }
 
-void FailingMTMS::flatten(Stream& stream)
+void FailingMTMS::flatten(Stream& stream) const
 {
     stream << _header << _mtms;
 }