diff --git a/sersendf.h b/sersendf.h
index 31385e1..22aa592 100644
--- a/sersendf.h
+++ b/sersendf.h
@@ -4,6 +4,6 @@
 #include	<avr/pgmspace.h>
 
 void sersendf(char *format, ...)		__attribute__ ((format (printf, 1, 2)));
-void sersendf_P(PGM_P format, ...);
+void sersendf_P(PGM_P format, ...)	__attribute__ ((format (printf, 1, 2)));
 
 #endif	/* _SERSENDF_H */