diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index 90d1254311..2f60fb569e 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -952,6 +952,7 @@ VALUE rb_mutex_unlock(VALUE mutex);
 VALUE rb_mutex_sleep(VALUE self, VALUE timeout);
 VALUE rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg);
 /* time.c */
+struct timespec;
 void rb_timespec_now(struct timespec *);
 VALUE rb_time_new(time_t, long);
 VALUE rb_time_nano_new(time_t, long);