diff --git a/otestpoint/labtools/delta.py b/otestpoint/labtools/delta.py
index 2ff6589..45f1096 100644
--- a/otestpoint/labtools/delta.py
+++ b/otestpoint/labtools/delta.py
@@ -205,6 +205,7 @@ def notify(self,subject):
                     # it match the current value structure
                     # (i.e. single value or list if values)
                     if previous != None and \
+                       data[timestamp][tag] != None and \
                        hasattr(previous, '__iter__') == hasattr(data[timestamp][tag], '__iter__') and \
                        pd.notnull(previous):
                         if data[timestamp][tag] >= previous: