diff --git a/product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/TaskConstraint/date_coherency_constraint.xml b/product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/TaskConstraint/date_coherency_constraint.xml
index 3f84c835fa05e118fa830ea4356b273c1292089b..9eee8116040c43c8ece9f51f522fc30f48a3299a 100644
--- a/product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/TaskConstraint/date_coherency_constraint.xml
+++ b/product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/TaskConstraint/date_coherency_constraint.xml
@@ -14,7 +14,7 @@
             <key> <string>expression</string> </key>
             <value> <string encoding="cdata"><![CDATA[
 
-python: object.getStopDate() >= object.getStartDate()
+python: object.getStopDate() >= object.getStartDate() if object.hasStartDate() else True
 
 ]]></string> </value>
         </item>