diff --git a/include/unit.h b/include/unit.h
index df31adca..b9fe96bc 100644
--- a/include/unit.h
+++ b/include/unit.h
@@ -32,6 +32,9 @@
  *     Markus Armbruster, 2006
  */
  
+#ifndef UNIT_H
+#define UNIT_H
+
 #include "empobj.h"
 
 struct ulist {
@@ -60,3 +63,5 @@ extern void unit_update_cargo(struct empobj *);
 extern void unit_drop_cargo(struct empobj *, natid);
 extern void unit_give_away(struct empobj *, natid, natid);
 extern void unit_wipe_orders(struct empobj *);
+
+#endif