diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index d39a9352c4..28d59d0cd6 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -408,6 +408,7 @@ VALUE rb_io_printf(int, VALUE*, VALUE);
 VALUE rb_io_print(int, VALUE*, VALUE);
 VALUE rb_io_puts(int, VALUE*, VALUE);
 VALUE rb_io_fdopen(int, int, const char*);
+VALUE rb_io_get_io(VALUE);
 VALUE rb_file_open(const char*, const char*);
 VALUE rb_file_open_str(VALUE, const char*);
 VALUE rb_gets(void);