Asp.Net Web Api - Returning 404 for IEnumerable<T> Get When null

I am creating an API using the release version of Asp.Net Web Api. I am trying to pass back the correct response code (404) if no results are found.

First Get Version (throws multiple enumeration error):

public IEnumerable<MyObjectType> Get(int id, string format)
{
    var db = new DbEntities();

    var result = db.pr_ApiSelectMyObjectType(store, id, format).AsEnumerable();
    if (result.Any())
    {
        return result;
    }
    var response = new HttpResponseMessage(HttpStatusCode.NotFound) 
        { Content = new StringContent("Unable to find any results") };
    throw new HttpResponseException(response);
}

Second Get Version (result is never null, so it always returns 200):

public IEnumerable<MyObject> Get(int id, string format)
{
    var db = new DbEntities();

    var result = db.pr_ApiSelectMyObjectType(store, id, format);
    if (result == null)
    {
        var response = new HttpResponseMessage(HttpStatusCode.NoContent) 
            { Content = new StringContent("Unable to find any results") };
        throw new HttpResponseException(response);
    }
    return result.AsEnumerable();
}

How do I pass back a 404 if no results are found? I know I could use a list, but I have a custom csv media type formatter that only works with IEnumerable types, so I would prefer to stick with that.