Comma seperated List in linq select

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LINQ

entity-framework

I have Table HR_Travel(TravelID, TravelCode....) and HR_TravelDocuments(TravelDocID, TravelID, DocUrl)

        FromDate = FromDate.AddDays(1);
        ToDate = ToDate.AddDays(1);
        List<dynamic> Lst = new List<dynamic>();

        var queryTravelDetails = from t in db.HR_TravelDetails
                                 where ((t.StatusDate >= FromDate && t.StatusDate <= ToDate)
                                 && (t.EmpID == EmpID || EmpID == 0) && (t.TravelStatus == TravelStatus || TravelStatus == "All"))
                                 orderby t.StatusDate descending
                                 select new 
                                 {
                                     TravelID = t.TravelID,
                                     TravelSubID = db.HR_TravelDetails.Where(i => i.TravelID == 0).FirstOrDefault().TravelID == null ? 0 : db.HR_TravelDetails.Where(i => i.TravelID == 0).FirstOrDefault().TravelID,
                                     t.TravelCode,
                                     t.EmpID,
                                     EmpName = db.EE_Employee.Where(i => i.EmpID == t.EmpID).FirstOrDefault().EmpName,
                                     t.CellNo,
                                     t.BoardingForm,
                                     t.DestinationTO,
                                     t.JournyDate,
                                     t.Purpose,
                                     t.Organization,
                                     t.TravelStatus,

                                     DocUrl = DocUrl = string.Join(",",( db.HR_TravelDocuments.Where(i => i.TravelID == t.TravelID && i.TravelSubID == 0).Select(i => i.DocUrl).ToList()))
                                 };
foreach (var element in queryTravelDetails) { Lst.Add(element);
}

Gives an error as

LINQ to Entities does not recognize the method 'System.String Join[String](System.String, System.Collections.Generic.IEnumerable`1[System.String])' method, and this method cannot be translated into a store expression.

Plese help me immediately