IQueryable to ToList then Cache results

Question

Hi Guys/Lady's , wondering can any one help me with the following problem/question.

Am trying to cache Iqueryable converted tolist<> & get data back out of the cache. However, can't work out why it is not working.

public class QDatalist
{
    public int q_id { get; set; }
    public string title { get; set; }
    public int? marks { get; set; }
}
private void bindquestions()
{        
    IQueryable<QDatalist> data;
    using (DataClassesDataContext db = new DataClassesDataContext())
    {
        if (Cache.Get("Qbank") == null)
        {
            data = db.tb_questions.Select(x => new QDatalist {marks = x.marks, q_id = x.q_id, title = x.title});
            Cache.Insert("Qbank", data.ToList<QDatalist>(), null, System.Web.Caching.Cache.NoAbsoluteExpiration, TimeSpan.FromMinutes(120));
        }
        else
        {
           List<QDatalist> M = (List<QDatalist>)Cache.Get("Qbank");
           data = M.AsQueryable<QDatalist>();                 
        }  
    }
}

Error am getting when try Get("Qbank") is:

System.InvalidCastException: [A]System.Collections.Generic.List1[member_Default+QDatalist] cannot be cast to [B]System.Collections.Generic.List1[member_Default+QDatalist]. Type A originates from 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\Windows\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll'. Type B originates from 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\Windows\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll'.

Can any one help? Thank you for your time, Chris