diff --git a/phpBB/includes/db/db_tools.php b/phpBB/includes/db/db_tools.php
index 819ef69c96..f4b181c6ad 100644
--- a/phpBB/includes/db/db_tools.php
+++ b/phpBB/includes/db/db_tools.php
@@ -611,7 +611,7 @@ class phpbb_db_tools
 	*	drop_columns: Removing/Dropping columns
 	*	add_primary_keys: adding primary keys
 	*	add_unique_index: adding an unique index
-	*	add_index: adding an index
+	*	add_index: adding an index (can be column:index_size if you need to provide size)
 	*
 	* The values are in this format:
 	*		{TABLE NAME}		=> array(
@@ -1804,6 +1804,12 @@ class phpbb_db_tools
 	{
 		$statements = array();
 
+		// remove index length unless MySQL4
+		if ('mysql_40' != $this->sql_layer)
+		{
+			$column = preg_replace('#:.*$#', '', $column);
+		}
+
 		switch ($this->sql_layer)
 		{
 			case 'firebird':
@@ -1814,6 +1820,16 @@ class phpbb_db_tools
 			break;
 
 			case 'mysql_40':
+				// add index size to definition as required by MySQL4
+				foreach ($column as $i => $col)
+				{
+					if (false !== strpos($col, ':'))
+					{
+						list($col, $index_size) = explode(':', $col);
+						$column[$i] = "$col($index_size)";
+					}
+				}
+			// no break
 			case 'mysql_41':
 				$statements[] = 'CREATE INDEX ' . $index_name . ' ON ' . $table_name . '(' . implode(', ', $column) . ')';
 			break;