diff --git a/src/if_mzsch.c b/src/if_mzsch.c
index c8be940..57b593a 100644
--- a/src/if_mzsch.c
+++ b/src/if_mzsch.c
@@ -139,7 +139,7 @@
 # define OUTPUT_LEN_TYPE long
 #endif
 static void do_output(char *mesg, OUTPUT_LEN_TYPE len);
-static void do_printf(char *format, ...);
+static void do_printf(char *format, ...) ATTRIBUTE_FORMAT_PRINTF(1, 2);
 static void do_flush(void);
 static Scheme_Object *_apply_thunk_catch_exceptions(
 	Scheme_Object *, Scheme_Object **);