diff --git a/phpBB/phpbb/db/driver/driver.php b/phpBB/phpbb/db/driver/driver.php
index f66f10322b..d2bb5f4f7c 100644
--- a/phpBB/phpbb/db/driver/driver.php
+++ b/phpBB/phpbb/db/driver/driver.php
@@ -831,7 +831,7 @@ abstract class driver implements driver_interface
 				break;
 				case 'NOT':
 
-					$condition = ' NOT ' . $this->_process_boolean_tree($condition);
+					$condition = ' NOT (' . $this->_process_boolean_tree($condition) . ') ';
 
 				break;