PHP Error while installing Invoiceplane 1.5.1 on a subdirectory

I tried to do a clean install of Invoiceplane 1.5.1 in a subdirectory on my server, I get to the menu where I can choose the language and then to the menu where it shows that my server fullfils all the Prerequisites, but then I’m always getting this error message:

A PHP Error was encountered

Severity: Notice

Message: Undefined property: CI::$mdl_settings

Filename: helpers/trans_helper.php

Line Number: 82

Backtrace:

File: /var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/application/helpers/trans_helper.php
Line: 82
Function: _error_handler

File: /var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/application/helpers/trans_helper.php
Line: 31
Function: reset_language

File: /var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/application/modules/setup/controllers/Setup.php
Line: 268
Function: trans

File: /var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/application/modules/setup/controllers/Setup.php
Line: 212
Function: check_database

File: /var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/index.php
Line: 334

Fatal error: Call to a member function setting() on null in

/var/www/vhosts/hosting2445.af919.netcup.net/httpdocs/invoice/application/helpers/trans_helper.php on line 82

A PHP Error was encountered

Severity: Error

Message: Call to a member function setting() on null

Filename: helpers/trans_helper.php

Line Number: 82

Backtrace:
Function: require_once

Okay, it works. I put in the setting of my database into index.php and then everything works fine (strange, because the text of the index.php says, that you don’t have to put your settings into it).