diff --git a/src/barrier/barrier.cpp b/src/barrier/barrier.cpp
index 8287b77d7..46d545938 100644
--- a/src/barrier/barrier.cpp
+++ b/src/barrier/barrier.cpp
@@ -113,6 +113,9 @@ Order VertexIndex<K>::order()
     return HALF_CALL(order());
 }
 
+// INSTANCES OF VERTEX INDEX
+template class VertexIndex<std::nullptr_t>;
+
 // **************** EdgeIndex
 template <class K>
 EdgeIterator EdgeIndex<K>::for_range(DbAccessor &t, Border<K> from,
@@ -133,6 +136,9 @@ Order EdgeIndex<K>::order()
     return HALF_CALL(order());
 }
 
+// INSTANCES OF EDGE INDEX
+template class EdgeIndex<std::nullptr_t>;
+
 // ************************* DbAccessor
 DESTRUCTOR(DbAccessor, DbAccessor);