Getting "Call to undefined function: form_select() " by choosing the 'spam tab'

I'm using the spam module from http://www.kerneltrap.org/jeremy/drupal/spam/

When I'm now choosing in /administer/trackback the tab spam, I'll get the message:

Fatal error: Call to undefined function: form_select() in /var/www/vhosts/raven.to/httpdocs/modules/trackback/trackback.module on line 780