diff --git a/firmware/common/sprintf.h b/firmware/common/sprintf.h
index be79683368..759715115b 100644
--- a/firmware/common/sprintf.h
+++ b/firmware/common/sprintf.h
@@ -20,6 +20,8 @@
 #ifndef __SPRINTF_H__
 #define __SPRINTF_H__
 
+#include <stddef.h>
+
 int snprintf (char *buf, size_t size, const char *fmt, ...);
 
 #endif /* __SPRINTF_H__ */