Invalid cast from 'System.String' to 'Telerik.Si

We have upgraded from 8.2 to 10.1 -we have fixed an  number of issues that typically happen with upgrades. We cannot solve this issue though.

We have a form that has two fields (first name, last name) we have a list of emails that are part of the NotificationEmailList. We add these emails to the formdata.

formEntry.PostedData.FormsData.Add(new KeyValuePair<string, object>("Description", NotificationEmailsList));

when we goto save:

formSubmition.Save(formEntry);

we get this error.

 Invalid cast from 'System.String' to 'Telerik.Sitefinity.Model.Lstring'.

Thanks in advance.