TruncateTime(System.Nullable`1[System.DateTime])' has no supported translation to SQL.'

My data class is

public class Data
{
    public DateTime? Date { get; set; }
    public int Count { get; set; } 
}

And the following query

var queryable = db.MemberStats.Where(ms => ms.MemberId == User.Identity.GetUserId())
                .GroupBy(n => TruncateTime(n.Commited))
                .Select(g => new Data()
                    {
                         Date = g.Key,
                         Count = g.Count()
                    }
                ).ToList();

returns

System.NotSupportedException: 'Method 'System.Nullable`1[System.DateTime] 
TruncateTime(System.Nullable`1[System.DateTime])' has no supported translation to SQL.'

What is wrong with that? How can i fix this one?

update: if i add ToList() before GroupBy() i have

   at System.Data.Entity.Core.Objects.EntityFunctions.TruncateTime(Nullable`1 dateValue)
   at .Controllers.ChartController.<>c.<Index>b__1_0(MemberStat n) in \Controllers\ChartController.cs:line 30
   at System.Linq.Lookup`2.Create[TSource](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at System.Linq.GroupedEnumerable`3.GetEnumerator()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Linq.SystemCore_EnumerableDebugView`1.get_Items()