diff --git a/Tracking/TrkDetDescr/TrkGeometry/TrkGeometry/LayerMaterialRecord.h b/Tracking/TrkDetDescr/TrkGeometry/TrkGeometry/LayerMaterialRecord.h
index 270e921d245a0da291f140a3ea623e596fe02ffb..d58cbc263d1f30dbdd33406a623c684842865007 100755
--- a/Tracking/TrkDetDescr/TrkGeometry/TrkGeometry/LayerMaterialRecord.h
+++ b/Tracking/TrkDetDescr/TrkGeometry/TrkGeometry/LayerMaterialRecord.h
@@ -118,7 +118,7 @@ namespace Trk {
         std::vector< std::vector< std::map<unsigned int, double> > > m_run_elements;
 
         //!< the final material properties                  
-        mutable MaterialPropertiesMatrix                             m_associatedLayerMaterial;
+        MaterialPropertiesMatrix                             m_associatedLayerMaterial;
                                                                      
         //!< clear the material -> calls delete
         void clearMaterial();