diff --git a/include/cyclone/collide_coarse.h b/include/cyclone/collide_coarse.h
index 8722169..e6b8d85 100644
--- a/include/cyclone/collide_coarse.h
+++ b/include/cyclone/collide_coarse.h
@@ -21,6 +21,7 @@
 #define CYCLONE_COLLISION_COARSE_H
 
 #include <vector>
+#include <cstddef>
 #include "contacts.h"
 
 namespace cyclone {