diff --git a/job.py b/job.py
index fc2d585..98b2443 100644
--- a/job.py
+++ b/job.py
@@ -135,7 +135,8 @@ def joblog(id):
     duration= duration-timedelta(microseconds=duration.microseconds)
     estimate=None
     duration_s = duration.total_seconds()
-    if duration_s > 300 and joblog.pa_job_state != "Completed":
+    # if job is old, not completed, and we have num_files and current_file_num > 0 so we can work out an estimated duration
+    if duration_s > 300 and joblog.pa_job_state != "Completed" and joblog.current_file_num and joblog.num_files:
         estimate_s = duration_s / joblog.current_file_num * joblog.num_files
         estimate = timedelta( seconds=(estimate_s-duration_s) )
         estimate = estimate - timedelta(microseconds=estimate.microseconds)