diff --git a/src/Plugin/views/filter/ContentLockFilter.php b/src/Plugin/views/filter/ContentLockFilter.php
index c0b7624..46f938f 100644
--- a/src/Plugin/views/filter/ContentLockFilter.php
+++ b/src/Plugin/views/filter/ContentLockFilter.php
@@ -19,10 +19,10 @@ class ContentLockFilter extends BooleanOperator {
   public function query() {
     $this->ensureMyTable();
     if (empty($this->value)) {
-      $this->query->addWhere($this->options['group'], $this->tableAlias . ".timestamp", "NULL", "=");
+			$this->query->addWhere($this->options['group'], $this->tableAlias . ".timestamp", 0, BooleanOperator::EQUAL);
     }
     else {
-      $this->query->addWhere($this->options['group'], $this->tableAlias . ".timestamp", "NULL", "<>");
+			$this->query->addWhere($this->options['group'], $this->tableAlias . ".timestamp", 0, BooleanOperator::NOT_EQUAL);
     }
   }