diff -u b/src/Plugin/monitoring/SensorPlugin/UserIntegritySensorPlugin.php b/src/Plugin/monitoring/SensorPlugin/UserIntegritySensorPlugin.php
--- b/src/Plugin/monitoring/SensorPlugin/UserIntegritySensorPlugin.php
+++ b/src/Plugin/monitoring/SensorPlugin/UserIntegritySensorPlugin.php
@@ -223,7 +223,7 @@
       $processed_users[$id]['mail'] = $user->getEmail();
       $processed_users[$id]['password'] = hash('sha256', $user->getPassword());
       $processed_users[$id]['changed'] = $user->getChangedTime();
-      $processed_users[$id]['last_accessed'] = ($user->getLastAccessedTime() != 0) ? $user->getLastAccessedTime() : null;
+      $processed_users[$id]['last_accessed'] = $user->getLastAccessedTime();
     }
     return $processed_users;
   }
@@ -258,7 +258,7 @@
       $row['expected_value'] = ['#markup' => ''];
 
       $row['time'] = ['#markup' => date("Y-m-d H:i:s", $time_stamp)];
-      $row['last_accessed'] = ['#markup' => isset($last_accessed) ? date("Y-m-d H:i:s", $last_accessed) : 'Never'];
+      $row['last_accessed'] = ['#markup' => $last_accessed != 0 ? date("Y-m-d H:i:s", $last_accessed) : 'never'];
       $rows[] = $row;
     }
 
@@ -280,7 +280,7 @@
         $row['expected_value'] = ['#markup' => $value['expected_value']];
 
         $row['time'] = ['#markup' => date("Y-m-d H:i:s", $time_stamp)];
-        $row['last_accessed'] = ['#markup' => date("Y-m-d H:i:s", $last_accessed)];
+        $row['last_accessed'] = ['#markup' => $last_accessed != 0 ? date("Y-m-d H:i:s", $last_accessed) : 'never'];
         $rows[] = $row;
       }
     }