Specified cast is not valid when getting Datatable Column Value

The above error message appears when I am trying to get Column Value from Datatable.

This is what I find in the stacktrace:

System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext()

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable1
source)

 and this in the TargetSite when debugging:
 

{ Boolean b__0(System.Data.DataRow)}

Here is my code:
DataTable hr = new DataTable();

            hr.Columns.Add("BookingDate");
             hr.Columns.Add("BookingId");
             hr.Columns.Add("BookingSource");
             hr.Columns.Add("CheckInDate");
             hr.Columns.Add("CheckOutDate");
 
 
             for (int i = 0; i < gmisc.GetModifiedBookings(gmoreq).Bookings.Length; i++)
             {
                 hr.Rows.Add();
                 hr.Rows[i]["BookingDate"] = Convert.ToDateTime(gmisc.GetModifiedBookings(gmoreq).Bookings[i].BookingDate.ToString());
                 hr.Rows[i]["BookingId"] = Convert.ToInt64(gmisc.GetModifiedBookings(gmoreq).Bookings[i].BookingId.ToString());
                 hr.Rows[i]["BookingSource"] = gmisc.GetModifiedBookings(gmoreq).Bookings[i].BookingSource.ToString();
                 hr.Rows[i]["CheckInDate"] = Convert.ToDateTime(gmisc.GetModifiedBookings(gmoreq).Bookings[i].CheckInDate.ToString());
                 hr.Rows[i]["CheckOutDate"] = Convert.ToDateTime(gmisc.GetModifiedBookings(gmoreq).Bookings[i].CheckOutDate.ToString());
 
 
 
             }
             Int64 BookingId = (from DataRow dr in hr.Rows
                           where (Int64)dr["BookingId"] == BookId
                           select (Int64)dr["BookingId"]).FirstOrDefault();
                 TextBox1.Text = Convert.ToString(BookingId);
 

Where did I go wrong, if somebody can please tell me.

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