diff --git a/phpBB/includes/db/db_tools.php b/phpBB/includes/db/db_tools.php
index 50e308dea2..d23323a5b1 100644
--- a/phpBB/includes/db/db_tools.php
+++ b/phpBB/includes/db/db_tools.php
@@ -2059,7 +2059,7 @@ class phpbb_db_tools
 		$table_prefix = substr(CONFIG_TABLE, 0, -6); // strlen(config)
 		if (strlen($table_name . $index_name) - strlen($table_prefix) > 24)
 		{
-			$max_length = $table_prefix + 24;
+			$max_length = strlen($table_prefix) + 24;
 			trigger_error("Index name '{$table_name}_$index_name' on table '$table_name' is too long. The maximum is $max_length characters.", E_USER_ERROR);
 		}
 
@@ -2096,7 +2096,7 @@ class phpbb_db_tools
 		$table_prefix = substr(CONFIG_TABLE, 0, -6); // strlen(config)
 		if (strlen($table_name . $index_name) - strlen($table_prefix) > 24)
 		{
-			$max_length = $table_prefix + 24;
+			$max_length = strlen($table_prefix) + 24;
 			trigger_error("Index name '{$table_name}_$index_name' on table '$table_name' is too long. The maximum is $max_length characters.", E_USER_ERROR);
 		}