diff --git a/extensions/openpower-pels/ascii_string.cpp b/extensions/openpower-pels/ascii_string.cpp
index 4bfc9cf..124bd0c 100644
--- a/extensions/openpower-pels/ascii_string.cpp
+++ b/extensions/openpower-pels/ascii_string.cpp
@@ -65,7 +65,7 @@
     }
 }
 
-void AsciiString::flatten(Stream& stream)
+void AsciiString::flatten(Stream& stream) const
 {
     stream.write(_string.data(), _string.size());
 }