--- ext/sdbm/_sdbm.c
+++ ext/sdbm/_sdbm.c
@@ -9,6 +9,7 @@
 
 #include "ruby/config.h"
 #include "ruby/defines.h"
+#include "ruby/ruby.h"
 
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>