HTML::Formhandler error: '...' is not a valid value

using HTML::Formhandler in Catalyst I defined a form based on a database table (item_class). What the form does is set the first three select lists based on the contents of several tables (other than the table on which the form is based) in the database. It looks like the select lists all get (html) correct options. However on commit I invariably get error: '...' is not a valid value ONLY for fields: prg_id and mrk_id.

What's my mistake? Any help is greatly appreciated.

My form definition:

package BskExt::Form::HMbestellingProducten;

use HTML::FormHandler::Moose;
extends 'HTML::FormHandler::Model::DBIC';
with 'HTML::FormHandler::Render::Table';

use namespace::autoclean;

has 'wpl_id' => ( is => 'rw' );

has '+item_class' => ( default => 'NewHmbestellingenproducten' );

has_field 'wklloc_id' => ( type => 'Select', label => 'Winkel(locatie)s' );
#has_field 'prg_id' => ( type => 'Select', label => 'Productgroep', label_column => 'naam', accessor => 'prg' );
has_field 'prg_id' => ( type => 'Select', label => 'Productgroep', error_messages => { select_invalid_value => undef } );
#has_field 'mrk_id' => ( type => 'Select', label => 'Merk', label_column => 'naam', accessor => 'mrk' );
has_field 'mrk_id' => ( type => 'Select', label => 'Merk' );
has_field 'productnaam' => ( type => 'Text',   label => 'Product *', size => 32, maxlength => 40, required => 1, messages => { required => 'Verplicht' }, validate_method => \&CatFormFieldValidate::validate_name_ext );
has_field ' ' => ( type => 'Submit', id => 'submit', value => 'Ga door' ); 

#
# The select list for wklloc_id is set by options_wklloc_id which has been omitted from
# this listing as it made the listing (much) too long
# 
sub options_mrk_id
{
  my ($self) = shift;

  my (@mrk_select_list);

  undef(@mrk_select_list);
  push @mrk_select_list, { value => '', label => 'Kies eerst een productgroep' };

  return @mrk_select_list;
}

sub options_prg_id
{
  my ($self) = shift;

  my (@prg_select_list);

  undef(@prg_select_list);
  push @prg_select_list, { value => '', label => 'Kies eerst een winkel(-locatie)' };

  return @prg_select_list;
}

My form template:

<script>
  $(function() {
    $("#wklloc_id").click(function() {
    $("#prg_id").load("WinkelLocatieProductgroepen?wklloc_id=" + $("#wklloc_id").val());
    });
  });

  $(function() {
    $("#prg_id").click(function() {
    $("#mrk_id").load("ProductgroepMerken?wklloc_id=" + $("#wklloc_id").val() + "&prg_id=" + $("#prg_id").val());
    });
  });
</script>

<p></p>
[%# Render the HTML::FormHandler Form %]
[% form.render %]

(The significant part of) my controller:

sub create : Chained('base') PathPart('create') Args(0) {
    my ($self, $c ) = @_;

    my $hmbestprod = $c->model('DB::NewHmbestellingenproducten')->new_result({});
    return $self->form($c, $hmbestprod,'new');
}

sub form
{
    my ( $self, $c, $hmbestprod, $action ) = @_;

    my ($form,$klant_info,$uc_action);

    $uc_action = uc($action);
    $klant_info = $c->model( 'BskExt' )->KlantInfo($c->session->{"klt_id"}||0);
    $form = BskExt::Form::HMbestellingProducten->new(wpl_id => $$klant_info{"wpl_id"});
    # Set the template
    $c->stash( template => 'hmbestellingproducten/form.tt2', action => $uc_action, form => $form );
    $form->process(
                    use_init_obj_over_item => 1,
                    item => $hmbestprod,
                    init_object => { "aantal" => 1 },
                    params => $c->req->params );
    return unless $form->validated;
    # Set a status message for the user & return to list
    $c->response->redirect($c->uri_for($self->action_for('list')));
}
#
# This helps jQuery to set the content of the ProductgroepMerken form list
#
sub ProductgroepMerken :Chained('base') :PathPart('ProductgroepMerken') :Args(0)
{
  my ($self, $c) = @_;

  my ($optstr,$prg_id,$productgroepmerken,$wklloc_id);

  $c->log->debug("ProductgroepMerken\n");

  $prg_id = $c->request->params->{"prg_id"};
  $wklloc_id = $c->request->params->{"wklloc_id"};
#
# The result: $productgroepmerken contains (at first glance) valid values. These values are not in any way (directly)
# related to the FORM item_class 'NewHmbestellingenproducten'
#
  $productgroepmerken = $c->model ( 'BskExt' )->ProductgroepMerken($wklloc_id,$prg_id);
  undef($optstr);
  foreach (@$productgroepmerken)
  {
      $optstr .= '<option value="'.$_->{"id"}.'">'.$_->{"naam"}.'</option>';
  }
  $c->log->debug("optstr= $optstr\n");
  $c->response->body($optstr);
}
#
# This helps jQuery to set the content of the WinkelLocatieProductgroepen form select list
#

sub WinkelLocatieProductgroepen :Chained('base') :PathPart('WinkelLocatieProductgroepen') :Args(0)
{
  my ($self, $c) = @_;

  my ($optstr,$productgroepen,$wklloc_id);

  $c->log->debug("WinkelLocatieProductgroepen\n");

  $wklloc_id = $c->request->params->{"wklloc_id"};
#
# The result: $productgroepen contains (at first glance) valid values. These values are not in any way (directly)
# related to the FORM item_class 'NewHmbestellingenproducten'
#
  $productgroepen = $c->model ( 'BskExt' )->WinkelLocatieProductgroepen($wklloc_id);
  $c->log->debug("productgroepen= \n".Dumper($productgroepen)."\n\n");
  undef($optstr);
  foreach (@$productgroepen)
  {
      $optstr .= '<option value="'.$_->{"id"}.'">'.$_->{"naam"}.'</option>';
  }
  $c->log->debug("optstr= $optstr\n");
  $c->response->body($optstr);
}