diff --git a/pkt-line.h b/pkt-line.h
index b2965869ad..450183b649 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -25,6 +25,7 @@ void packet_buf_flush(struct strbuf *buf);
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 int packet_flush_gently(int fd);
 int packet_write_fmt_gently(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
+LAST_ARG_MUST_BE_NULL
 int packet_writel(int fd, const char *line, ...);
 int write_packetized_from_fd(int fd_in, int fd_out);
 int write_packetized_from_buf(const char *src_in, size_t len, int fd_out);