diff --git a/phpBB/includes/functions.php b/phpBB/includes/functions.php
index b8ea80ad4a..ceaf426850 100644
--- a/phpBB/includes/functions.php
+++ b/phpBB/includes/functions.php
@@ -175,7 +175,8 @@ function set_config_count($config_name, $increment, $is_dynamic = false)
 	switch ($db->sql_layer)
 	{
 		case 'firebird':
-			$sql_update = 'CAST(CAST(config_value as DECIMAL(255, 0)) + ' . (int) $increment . ' as VARCHAR(255))';
+			// Precision must be from 1 to 18
+			$sql_update = 'CAST(CAST(config_value as DECIMAL(18, 0)) + ' . (int) $increment . ' as VARCHAR(255))';
 		break;
 
 		case 'postgres':