diff --git a/lib/punchcard.rb b/lib/punchcard.rb
index 8015631..5ac0341 100755
--- a/lib/punchcard.rb
+++ b/lib/punchcard.rb
@@ -121,6 +121,7 @@ def csv(start_at: nil, end_at: nil)
         start_time: start_time,
         end_time: end_time
       )
+      next if start_time.nil? || end_time.nil?
 
       last_activity = points[1] || points[0]
       durations.push end_time - start_time