diff --git a/marshal.c b/marshal.c
index 389005eb22..5acc018ddc 100644
--- a/marshal.c
+++ b/marshal.c
@@ -13,6 +13,7 @@
 # error too old GCC
 #endif
 
+#include "ruby/ruby.h"
 #include "ruby/io.h"
 #include "internal.h"
 #include "ruby/st.h"