HepMC/GenEvent.h  |    1 +
 HepMC/GenVertex.h |    1 +
 2 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/HepMC/GenEvent.h b/HepMC/GenEvent.h
index 72374cb..d10b9a0 100644
--- a/HepMC/GenEvent.h
+++ b/HepMC/GenEvent.h
@@ -137,6 +137,7 @@ namespace HepMC {
 #include <vector>
 #include <algorithm>
 #include <iostream>
+#include <cstddef>
 
 namespace HepMC {
 
diff --git a/HepMC/GenVertex.h b/HepMC/GenVertex.h
index 2f3d4de..fc3bf63 100644
--- a/HepMC/GenVertex.h
+++ b/HepMC/GenVertex.h
@@ -29,6 +29,7 @@
 #include <vector>
 #include <set>
 #include <algorithm>
+#include <cstddef>
 
 namespace HepMC {