diff --git a/server/completion.c b/server/completion.c
index f024b44a076f6c5a5e4dec83278d4cc793fb9466..1282bb47d93b0f7446dd2cec0ed06f4370c9b5e5 100644
--- a/server/completion.c
+++ b/server/completion.c
@@ -29,6 +29,7 @@
 #include "config.h"
 #include "wine/port.h"
 
+#include <stdarg.h>
 #include <stdio.h>
 
 #include "ntstatus.h"