This happens on a hidden field.

   at Umbraco.Forms.Core.Providers.FieldTypes.HiddenField.get_Values() in d:\TeamCity6\buildAgent\work\278191fcc80f7b16\Umbraco.Forms.Core\Providers\FieldTypes\HiddenField.cs:line 63
   at Umbraco.Forms.Core.RecordField.CreateFromPage(Page page, Record record) in d:\TeamCity6\buildAgent\work\278191fcc80f7b16\Umbraco.Forms.Core\RecordField.cs:line 151
   at BSessions.GroupWeb.UserControls.InvitationImport.Button1_Click(Object sender, EventArgs e) in C:\sourcecode\GroupWeb\BSessions.GroupWeb\UserControls\InvitationImport.ascx.cs:line 114
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Record record = Umbraco.Forms.Core.Record.Create();

record.Created = DateTime.Now;
record.UmbracoPageId = umbraco.presentation.UmbracoContext.Current.PageId.Value;
record.IP = Request.UserHostAddress;
record.Form = form.Id;
record.CurrentPage = form.Pages[0].Id;
record.MemberKey = umbraco.cms.businesslogic.member.Member.GetCurrentMember().Id;

store.InsertRecord(record, form);

List<RecordField> fields = Umbraco.Forms.Core.RecordField.CreateFromPage(form.Pages[0], record); 

Is CreateFromPage deprecated, i.e. I should create every single field manually or is it something wrong with the hidden field? 

If I'm not supposed to use CreateFromPage I would very much like to see an example on how this should be done.

Thanks,
Harald