diff --git a/examples/DDG4/src/ChannelingCrystalMaterial.cpp b/examples/DDG4/src/ChannelingCrystalMaterial.cpp
index fe8a134d9372614d1ee7b80cba734866ea581ecd..8bd416295741b9cc8e8b3cdeffede7d9d88e889a 100644
--- a/examples/DDG4/src/ChannelingCrystalMaterial.cpp
+++ b/examples/DDG4/src/ChannelingCrystalMaterial.cpp
@@ -64,7 +64,7 @@ namespace dd4hep   {
     channelingData->SetFilename(crystal_data);
 
     /// Set crystal orientation
-    orientation = mat.propertyRef("Geant4-ignore:crystal_orientation");
+    orientation = mat.constPropertyRef("Geant4-ignore:crystal_orientation");
     if ( !orientation.empty() )   {
       Direction direction;
       Property  prop(direction);