I was trying to solve this problem projecteuler,problem125

this is my solution in python(just for understanding the logic)

import math

lim=10**8
found=set()
for start in xrange(1,int(math.sqrt(lim))):
    sos = start*start
    for i in xrange(start+1,int(math.sqrt(lim))):
        sos += (i*i)
        if sos >= lim: break
        s=str(int(sos))
        if s==s[::-1]:
            found.add(sos)

print sum(found)

the same code I wrote in C# is as follows

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        public static bool isPalindrome(string s)
        {
            string temp = "";
            for (int i=s.Length-1;i>=0;i-=1){temp+=s[i];}
            return (temp == s);
        }
        static void Main(string[] args)
        {
            int lim = Convert.ToInt32(Math.Pow(10,8));
            var found = new HashSet<int>();
            for (int start = 1; start < Math.Sqrt(lim); start += 1)
            {
                int s = start *start;
                for (int i = start + 1; start < Math.Sqrt(lim); i += 1)
                {
                    s += i * i;
                    if (s > lim) { break; }
                    if (isPalindrome(s.ToString())) 
                    { found.Add(s); }
                }
            }
            Console.WriteLine(found.Sum());

        }
    }
}

the code debugs fine until it gives an exception at Console.WriteLine(found.Sum()); (line31). Why can't I find Sum() of the set found