A workaround that will recognise when a SingleResult type does not contain any results and replace it with a 404 instead of throwing a SerializationException.

ReplaceNullContentWithNotFoundAttribute

        internal class ReplaceNullContentWithNotFoundAttribute : EnableQueryAttribute
        {
            public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
            {
                base.OnActionExecuted(actionExecutedContext);

                HttpResponseMessage httpResponseMessage = actionExecutedContext.Response;
                if (httpResponseMessage.IsSuccessStatusCode && IsContentMissingValue(httpResponseMessage))
                {
                    actionExecutedContext.Response = actionExecutedContext.Request.CreateResponse(HttpStatusCode.NotFound);
                }
            }

            private static bool IsContentMissingValue(HttpResponseMessage httpResponseMessage)
            {
                var objectContent = httpResponseMessage.Content as ObjectContent;
                if (objectContent == null)
                {
                    return false;
                }

                var type = GetType(objectContent);
                return type == typeof(SingleResult<>) && objectContent.Value == null;
            }

            private static Type GetType(ObjectContent objectContent)
            {
                var type = objectContent.ObjectType;
                if (type.IsGenericType && !type.IsGenericTypeDefinition)
                {
                    var genericTypeDefinition = type.GetGenericTypeDefinition();
                    type = genericTypeDefinition;
                }
                return type;
            }
        }

Great solution. The official workaround (http://odata.github.io/WebApi/#10-02-work-around-for-singleresult-create) requires including OWin libraries, which seems a bit of an overkill. And using Web API filters is cleaner anyways.

If you wish to produce an OData error response instead of an empty 404 response, use CreateErrorResponse extension method:

actionExecutedContext.Response = actionExecutedContext.Request.CreateErrorResponse(HttpStatusCode.NotFound, 
    HttpWorkerRequest.GetStatusDescription((int)HttpStatusCode.NotFound));

@andygjp I saw your comment on SO and I dont have enough rep there to ask you how to implement your code in a simple get call like this, also @taehtinen

    [EnableQuery]
    public SingleResult<Apple> Get([FromODataUri] int key)
    {
        IQueryable<Apple> result = db.Apples.Where(p => p.AppleId == key);
        return SingleResult.Create(result);
    }