diff --git a/firmware/common/sprintf.h b/firmware/common/sprintf.h
index 759715115b..ecbbc8a419 100644
--- a/firmware/common/sprintf.h
+++ b/firmware/common/sprintf.h
@@ -21,7 +21,9 @@
 #define __SPRINTF_H__
 
 #include <stddef.h>
+#include <stdarg.h>
 
 int snprintf (char *buf, size_t size, const char *fmt, ...);
+int vsnprintf (char *buf, int size, const char *fmt, va_list ap);
 
 #endif /* __SPRINTF_H__ */