Cannot convert IQueryable<> to IOrderedQueryable error

I have the following LINQ code:

    var posts = (from p in db.Posts
         .Include("Site")
         .Include("PostStatus")
        where p.Public == false
        orderby p.PublicationTime 
        select p);

        if (!chkShowIgnored.Checked) {
            posts = posts.Where(p => p.PostStatus.Id != 90);
        }

That last line (the extra where) is giving me the error:

Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Linq.IOrderedQueryable'.

I'm not sure what this means...
Why am I getting this error?
It appeared once I added the "orderby" clause to the query, before that it compiled fine, so I have kind of a hunch of what is going on, but I can't quite put my finger into it.