diff --git a/mailmap.c b/mailmap.c
index d1f7c0d..462b395 100644
--- a/mailmap.c
+++ b/mailmap.c
@@ -8,6 +8,7 @@
 #define debug_mm(...) fprintf(stderr, __VA_ARGS__)
 #define debug_str(X) ((X) ? (X) : "(none)")
 #else
+__attribute__((format (printf, 1, 2)))
 static inline void debug_mm(const char *format, ...) {}
 static inline const char *debug_str(const char *s) { return s; }
 #endif