C# AsParallel can't reentry

namespace ParallelTest {
    class Program {

        static IEnumerable<string> AllCombinationsNoRepeat(string prefix, int length) {
            return
                length == 0
                ? new[] { prefix }
                : new[] { prefix }.Concat(
                    Enumerable.Range(0, 9)
                        //.AsParallel() //FIXME: Se debe quedar deadlocked esperando un worker thread libre...
                        .Select(n => prefix+n)                                          //Append char
                        .Where(s => s.GroupBy(ch=>ch).All(g=>g.Take(2).Count() < 2))    //No chars repeated
                        .SelectMany(s => AllCombinationsNoRepeat(s, length-1))          //Combine recursively
                )
                ;
        }

        static void Main(string[] args) {
            foreach(var x in AllCombinationsNoRepeat("", 9))
                Console.WriteLine(x);

            Console.ReadKey();

        }
    }
}