[customer.module] erp_filter_form function does not exist

Click on "customer list" the error

Fatal error: Call to undefined function: erp_filter_form() in /home/olivier/public_html/drupal-erp/customer/customer.module on line 794

appears. The cause is:

$form = erp_filter_form($edit);

Just changing it to

$form = $edit;

Seems to resolve the issue, but I don't know if that is the intended behaviour. Could someone enlighten me?