diff --git a/ext/-test-/dln/empty/empty.c b/ext/-test-/dln/empty/empty.c
index c4f94f1644..2b4fd42cd2 100644
--- a/ext/-test-/dln/empty/empty.c
+++ b/ext/-test-/dln/empty/empty.c
@@ -1,3 +1,5 @@
+#include "ruby.h"
+
 void
 Init_empty(void)
 {