Null linq query issue - web api

[miss786]

Dear all, I am having a little issue with calling couple of queries from my filter method and would like to seek some a guidance to what I may doing wrong. if I call (api/test?tag=##,##,...), the query works fine. However, if I do not use the line of code -- var data2 = db.data_qy.AsQueryable(); and I call ((api/test?price_type=BVR), the query works and result is shown, otherwise if I do not use that line of code, i get a error -- ExceptionMessage":"Value cannot be null Also, If I use var data2 = db.data_qy.AsQueryable(); and call query ((api/test?price_type=BVR&deal_type=TYP), the query outputs entire dataset, instead filtering result to the search query. Please help. Many thanks

public HttpResponseMessage get([FromUri] Query query)
{
IQueryable data = null;

        if (!string.IsNullOrEmpty(query.tag))
        {
            var ids = query.tag.Split(',');

            var dataMatchingTags = db.data\_qy.Where(c => ids.Any(id => c.TAG.Contains(id)));

            if (data == null)
                data = dataMatchingTags;
            else
                data = data.Union(dataMatchingTags);
        }

        var data2 = db.data\_qy.AsQueryable();

        if (query.deal\_type != null)
        {
            data = data2.Where(c => c.Type == query.deal\_type);
        }

        if (query.price\_type != null)
        {
            data = data2.Where(c => c.Cover == query.price\_type);
        }

        var materializedData = data.ToList();

        if (!materializedData.Any())
        {
            var message = string.Format("No data found");
            return Request.CreateErrorResponse(HttpStatusCode.NotFound, message);
        }
        return Request.CreateResponse(HttpStatusCode.OK, materializedData);
    }