diff --git a/src/builtin.cmod b/src/builtin.cmod
index dd1835bb437168174fd30226dd3a06e1405f518f..381549596a92f6e68bc354287abebc31e302a307 100644
--- a/src/builtin.cmod
+++ b/src/builtin.cmod
@@ -1,3 +1,5 @@
+/* -*- c -*- */
+
 #include "global.h"
 #include "interpret.h"
 #include "svalue.h"