diff --git a/auth2-kbdint.c b/auth2-kbdint.c
index 9015964..a4fc9e6 100644
--- a/auth2-kbdint.c
+++ b/auth2-kbdint.c
@@ -27,6 +27,8 @@
 
 #include <sys/types.h>
 
+#include <stdarg.h>
+
 #include "xmalloc.h"
 #include "packet.h"
 #include "key.h"