[Solved]-Enumerable.Empty<T>().AsQueryable(); This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code-LINQ,C#

you can refactor the code to not need an empty set by only ever unioning results that you have:

public static iqueryable<t> applysearch<t>(this iqueryable<t> queryable, searchmodel search) where t : class 
{
    var subqueries = new list<iqueryable<t>>();
    if (search != null)
    {
        if (search.policynumber.hasvalue && typeof (ipolicynumber).isassignablefrom(queryable.elementtype))
        {
            subqueries.add(queryable.searchbypolicynumber(search));
        }

        if (search.uniqueid.hasvalue && typeof (iuniqueid).isassignablefrom(queryable.elementtype))
        {
            subqueries.add(queryable.searchbyuniqueid(search));
        }

        if (!string.isnullorwhitespace(search.postcode) && typeof(ipostcode).isassignablefrom(queryable.elementtype))
        {
            subqueries.add(queryable.searchbypostcode(search));
        }
    }

    return subqueries.defaultifempty(queryable)
        .aggregate((a, b) => a.union(b));
}