diff --git a/app/controllers/course/timesrooms.php b/app/controllers/course/timesrooms.php
index 3d55469a8c331152bd1dbf6e6903b1ba58526457..52fa6fff31710bed313fb702687e8431665ff5e9 100644
--- a/app/controllers/course/timesrooms.php
+++ b/app/controllers/course/timesrooms.php
@@ -771,7 +771,8 @@ class Course_TimesroomsController extends AuthenticatedController
     public function stack_action($cycle_id = '')
     {
         $_SESSION['_checked_dates'] = Request::optionArray('single_dates');
-        if (empty($_SESSION['_checked_dates']) && isset($_SESSION['_checked_dates'])) {
+        $_SESSION['_checked_dates'] = $this->validateDateIds($_SESSION['_checked_dates']);
+        if (count($_SESSION['_checked_dates']) === 0) {
             PageLayout::postError(_('Sie haben keine Termine ausgewählt!'));
             $this->redirect('course/timesrooms/index', ['contentbox_open' => $cycle_id]);
 
@@ -1874,4 +1875,35 @@ class Course_TimesroomsController extends AuthenticatedController
             }
         }
     }
+
+    private function validateDateIds(array $date_ids): array
+    {
+        if (count($date_ids) === 0) {
+            return [];
+        }
+
+        $valid = [];
+
+        CourseDate::findEachBySQL(
+            function (CourseDate $date) use (&$valid) {
+                if ($date->range_id === $this->course_id) {
+                    $valid[] = $date->id;
+                }
+            },
+            'range_id = ? AND termin_id IN (?)',
+            [$this->course->id, $date_ids]
+        );
+
+        CourseExDate::findEachBySQL(
+            function (CourseExDate $date) use (&$valid) {
+                if ($date->range_id === $this->course_id) {
+                    $valid[] = $date->id;
+                }
+            },
+            'range_id = ? AND termin_id IN (?)',
+            [$this->course->id, $date_ids]
+        );
+
+        return $valid;
+    }
 }