diff --git a/Combinatorial_map/include/CGAL/Compact_container_with_index.h b/Combinatorial_map/include/CGAL/Compact_container_with_index.h
index c6266330581..37d555793b6 100644
--- a/Combinatorial_map/include/CGAL/Compact_container_with_index.h
+++ b/Combinatorial_map/include/CGAL/Compact_container_with_index.h
@@ -448,7 +448,7 @@ public:
   {}
 
   /// Get the underlying index
-  operator size_t() const
+  operator size_type() const
   { return m_idx; }
 
   // Constructor allowing to transform an index from one container to another