Call to undefined function get_domain() - conStruct.install line 36 on module activation

When i enable the two minimal needed modules
conStruct Browse Tool
conStruct Core

i got sth like:

Call to undefined function get_domain()

this function seems to be unknown, grepped in complete drupal

// variable_set("conStruct_UrisDomain", str_replace("http://", "", get_domain($normalized_base_url)));

just replaced it hardcoded as workaround

variable_set("conStruct_UrisDomain", 'domain.net');

PHP 5.2.6 drupal-6.13