diff --git a/lib/cli.py b/lib/cli.py
index 71d010f713545d4171db0d78e93d5ca27e775198..71a99530708126aaec0c826b01a16c5acf04d861 100644
--- a/lib/cli.py
+++ b/lib/cli.py
@@ -703,5 +703,7 @@ def FormatTimestamp(ts):
   @returns: a string with the formatted timestamp
 
   """
+  if not isinstance (ts, (tuple, list)) or len(ts) != 2:
+    return '?'
   sec, usec = ts
   return time.strftime("%F %T", time.localtime(sec)) + ".%06d" % usec