diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 32ab94cd06b43cd05316f9a16981adc883c0d066..dbfe14f3fec3f8122e4855727bd7d1c3e4b3073e 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -49,15 +49,14 @@ int cmd_upload_archive_writer(int argc, const char **argv, const char *prefix)
 __attribute__((format (printf, 1, 2)))
 static void error_clnt(const char *fmt, ...)
 {
-	char buf[1024];
+	struct strbuf buf = STRBUF_INIT;
 	va_list params;
-	int len;
 
 	va_start(params, fmt);
-	len = vsprintf(buf, fmt, params);
+	strbuf_vaddf(&buf, fmt, params);
 	va_end(params);
-	send_sideband(1, 3, buf, len, LARGE_PACKET_MAX);
-	die("sent error to the client: %s", buf);
+	send_sideband(1, 3, buf.buf, buf.len, LARGE_PACKET_MAX);
+	die("sent error to the client: %s", buf.buf);
 }
 
 static ssize_t process_input(int child_fd, int band)