diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 2caedf1..dc872f6 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -88,11 +88,11 @@ int cmd_upload_archive(int argc, const char **argv, const char *prefix)
 	writer.git_cmd = 1;
 	if (start_command(&writer)) {
 		int err = errno;
-		packet_write(1, "NACK unable to spawn subprocess\n");
+		packet_write_fmt(1, "NACK unable to spawn subprocess\n");
 		die("upload-archive: %s", strerror(err));
 	}
 
-	packet_write(1, "ACK\n");
+	packet_write_fmt(1, "ACK\n");
 	packet_flush(1);
 
 	while (1) {