diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 6e9ef35a43a7..af400d083777 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -202,7 +202,7 @@ struct ffs_epfile {
 struct ffs_buffer {
 	size_t length;
 	char *data;
-	char storage[];
+	char storage[] __counted_by(length);
 };
 
 /*  ffs_io_data structure ***************************************************/