diff --git a/src/include/gpxe/open.h b/src/include/gpxe/open.h
index abba29c4..beab0a1f 100644
--- a/src/include/gpxe/open.h
+++ b/src/include/gpxe/open.h
@@ -7,6 +7,7 @@
  *
  */
 
+#include <stdarg.h>
 #include <gpxe/tables.h>
 
 struct xfer_interface;