diff --git a/modules/npdm/externalsort.h b/modules/npdm/externalsort.h
index e07c5b8..d84e457 100644
--- a/modules/npdm/externalsort.h
+++ b/modules/npdm/externalsort.h
@@ -7,6 +7,7 @@
 #include <boost/filesystem.hpp>
 #include <boost/archive/binary_oarchive.hpp>
 #include <boost/archive/binary_iarchive.hpp>
+#include <boost/serialization/vector.hpp>
 #include <vector> 
 #include <fstream>
 #include <iostream>