diff --git a/Geo/MElement.h b/Geo/MElement.h
index 76429591a1bec9ae7a315d4e3ba2d86c91941fa9..8258a1f5ff250669f1b9a95307e076bcf2d8c9c2 100644
--- a/Geo/MElement.h
+++ b/Geo/MElement.h
@@ -110,7 +110,7 @@ class MElement
   virtual int getNumVolumeVertices() const { return 0; }
 
   // get the number of primary vertices (first-order element)
-  int getNumPrimaryVertices()
+  int getNumPrimaryVertices() const
   {
     return getNumVertices() - getNumEdgeVertices() - getNumFaceVertices() -
       getNumVolumeVertices();