When using a custom FieldSettingType on a custom FieldType i get an exception.

When I add the custom FieldType to a form the setting are working.
But after the form has been saved in can not be opened since an exception are being thrown.

It seems like it can not set the custom FieldSettingType

Here is the exception

[MissingMethodException: Method 'Upac.CampaignMonitor.ContourExtensions.ListsFieldType.Lists to include' not found.]
   System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams) +2172
   Umbraco.Forms.Core.FieldType.LoadSettings(List`1 settings) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\FieldType.cs:86
   Umbraco.Forms.Data.Storage.FieldStorage.GetAllFields(FieldSet fieldset) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Data\FormStorage\FieldStorage.cs:96
   Umbraco.Forms.Data.Storage.FieldSetStorage.GetAllFieldSets(Page page) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Data\FormStorage\FieldSetStorage.cs:42
   Umbraco.Forms.Data.Storage.PageStorage.GetAllPages(Form form) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Data\FormStorage\PageStorage.cs:41
   Umbraco.Forms.Data.Storage.<>c__DisplayClass9.<GetForm>b__8() in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Data\FormStorage\FormStorage.cs:133
   umbraco.cms.businesslogic.cache.Cache.GetCacheItem(String cacheKey, Object syncLock, CacheItemPriority priority, CacheItemRemovedCallback refreshAction, CacheDependency cacheDependency, TimeSpan timeout, GetCacheItemDelegate`1 getCacheItem) +213
   umbraco.cms.businesslogic.cache.Cache.GetCacheItem(String cacheKey, Object syncLock, CacheItemPriority priority, CacheItemRemovedCallback refreshAction, TimeSpan timeout, GetCacheItemDelegate`1 getCacheItem) +121
   umbraco.cms.businesslogic.cache.Cache.GetCacheItem(String cacheKey, Object syncLock, CacheItemRemovedCallback refreshAction, TimeSpan timeout, GetCacheItemDelegate`1 getCacheItem) +118
   Umbraco.Forms.Core.Services.CacheService.GetCacheItem(String cacheKey, Object syncLock, TimeSpan timeout, GetCacheItemDelegate`1 getCacheItem) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Services\CacheService\CacheService.cs:62
   Umbraco.Forms.Data.Storage.FormStorage.GetForm(Guid id) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.Core\Data\FormStorage\FormStorage.cs:124
   Umbraco.Forms.UI.Pages.editForm.Page_Load(Object sender, EventArgs e) in d:\TeamCity\buildAgent\work\e4473b9ec9597356\Umbraco.Forms.UI\Pages\editForm.aspx.cs:32
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   umbraco.BasePages.BasePage.OnLoad(EventArgs e) +49
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627

BTW the custom FieldSettingType works perfectly on my custom FieldPreValueSourceType

The source code

namespace Upac.CampaignMonitor.ContourExtensions
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web.UI.WebControls;

    using CampaignMonitorAPIWrapper.CampaignMonitorAPI;

    using Umbraco.Forms.Core;
    using Umbraco.Forms.Core.Controls;

    public class ListsFieldType : FieldType
    {
        #region Fields

        private ValidateableCheckBoxList cbl;
        private List<object> value;

        #endregion Fields

        #region Constructors

        public ListsFieldType()
        {
            base.Id = new Guid("{46172748-DF31-4c14-B4F3-300E355606A8}");
            base.Name = "Campaign Monitor Lists";
            base.Description = "Renders a checkbox list with mailinglists";
            this.Icon = "checkboxlist.png";
            this.DataType = FieldDataType.String;
            this.value = new List<object>();
        }

        #endregion Constructors

        #region Properties

        public override WebControl Editor
        {
            get
            {
                cbl = new ValidateableCheckBoxList();
                cbl.CssClass = "checkboxlist";
                cbl.RepeatLayout = RepeatLayout.Flow;

                ListItem[] items = GetListItems();
                cbl.Items.AddRange(items);
                return cbl;
            }
            set
            {
                base.Editor = value;
            }
        }

        [Umbraco.Forms.Core.Attributes.Setting("Lists to include", description = "The selected lists the user can subscribe to", control = "Upac.CampaignMonitor.ContourExtensions.ListsFieldSetting", assembly = "Upac.CampaignMonitor")]
        public string ListsToInclude
        {
            get;
            set;
        }

        public override List<object> Values
        {
            get
            {
                if (cbl != null && cbl.SelectedIndex > -1)
                {
                    value.Clear();
                    foreach (ListItem item in cbl.Items)
                    {
                        if (item.Selected)
                        {
                            value.Add(item.Value);
                        }
                    }
                }
                return value;
            }
            set
            {
                this.value = value;
            }
        }

        #endregion Properties

        #region Methods

        public override string RenderPreview()
        {
            return "<div class=\"checkboxlist\"></div>";
            // More to come
        }

        public override string RenderPreviewWithPrevalues(List<object> prevalues)
        {
            return "<div class=\"checkboxlist\"></div>";
            // More to come
        }

        private ListItem[] GetListItems()
        {
            List<ListItem> items = new List<ListItem>();
            if (!string.IsNullOrEmpty(ListsToInclude))
            {
                string[] selectedValues = ListsToInclude.Split(",".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
                Api api = new Api();
                List<List> lists = api.GetLists();
                foreach (string selectedValue in selectedValues)
                {
                    List firstOrDefault = lists.FirstOrDefault(s => s.ListID == selectedValue);
                    if (firstOrDefault != null)
                    {
                        ListItem listItem = new ListItem(firstOrDefault.Name, firstOrDefault.ListID)
                        {
                            Selected = value.Contains(firstOrDefault.ListID)
                        };
                        items.Add(listItem);
                    }
                }
            }
            return items.ToArray();
        }

        #endregion Methods
    }
}