IDbSet and Exposing Include method via Extension Method

I am using the Code-First approach with EF and I wanted to use IDbSet instead of DbSet so I could do Unit Testing with mocks. My problem is I use the Include() method for eager loading where necessary but Include() is not exposed via IDbSet. I saw an example code using an extension method to expose Include() but it doesn't seem to work for me; the objectQuery object in this example is always null. Please let me know how to fix this.

public static class IQueryableExtension
{
    public static IQueryable<T> Include<T>(this IQueryable<T> source, string path)
        where T : class
    {
        ObjectQuery<T> objectQuery = source as ObjectQuery<T>;
        if (objectQuery != null)
        {
            return objectQuery.Include(path);
        }
        return source;
    }

    public static IQueryable<T> Include<T, TProperty>(this IQueryable<T> source, 
        System.Linq.Expressions.Expression<Func<T, TProperty>> path)
        where T : class
    {
        ObjectQuery<T> objectQuery = source as ObjectQuery<T>;
        if (objectQuery != null)
        {
            return source.Include(path);
        }
        return source;
    }
}