diff --git a/sao/src/view/calendar.js b/sao/src/view/calendar.js
index 51ae25784b..0800848fcb 100644
--- a/sao/src/view/calendar.js
+++ b/sao/src/view/calendar.js
@@ -309,13 +309,18 @@
             var last_datetime = Sao.DateTime(this.end);
             var dtstart = this.attributes.dtstart;
             var dtend = this.attributes.dtend || dtstart;
-            return ['OR',
+            return [
+                [dtstart, '!=', null],
+                [dtend, '!=', null],
+                ['OR',
                     ['AND', [dtstart, '>=', first_datetime],
                         [dtstart,  '<',  last_datetime]],
                     ['AND', [dtend, '>=', first_datetime],
                         [dtend, '<', last_datetime]],
                     ['AND',  [dtstart, '<', first_datetime],
-                        [dtend, '>', last_datetime]]];
+                        [dtend, '>', last_datetime]],
+                ],
+            ];
         },
         get_displayed_period: function(){
             var DatesPeriod = [];
diff --git a/tryton/tryton/gui/window/view_form/view/calendar_gtk/calendar_.py b/tryton/tryton/gui/window/view_form/view/calendar_gtk/calendar_.py
index fc3f3e7539..af9dc0db0d 100644
--- a/tryton/tryton/gui/window/view_form/view/calendar_gtk/calendar_.py
+++ b/tryton/tryton/gui/window/view_form/view/calendar_gtk/calendar_.py
@@ -65,13 +65,18 @@ class Calendar_(goocalendar.Calendar):
             self.current_domain_period.get_dates(True)
         dtstart = self.attrs['dtstart']
         dtend = self.attrs.get('dtend') or dtstart
-        domain = ['OR',
-            ['AND', (dtstart, '>=', first_datetime),
-                (dtstart, '<', last_datetime)],
-            ['AND', (dtend, '>=', first_datetime),
-                (dtend, '<', last_datetime)],
-            ['AND', (dtstart, '<', first_datetime),
-                (dtend, '>', last_datetime)]]
+        domain = [
+            (dtstart, '!=', None),
+            (dtend, '!=', None),
+            ['OR',
+                ['AND', (dtstart, '>=', first_datetime),
+                    (dtstart, '<', last_datetime)],
+                ['AND', (dtend, '>=', first_datetime),
+                    (dtend, '<', last_datetime)],
+                ['AND', (dtstart, '<', first_datetime),
+                    (dtend, '>', last_datetime)],
+                ],
+            ]
         return domain
 
     def get_colors(self, record):